# Patent

## Title
Method of measuring thin layers using SIMS

## Abstract

A method for measuring the thickness of a layer is provided, comprising (a) providing a structure (101) comprising a first layer disposed on a second layer; (b) impinging (103) the structure with a first ion beam comprising a first isotope, thereby sputtering off a portion of the first layer which contains a second isotope and exposing a portion of the second layer; and (c) determining (105) the thickness of the first layer by measuring the amount of the second isotope which is sputtered off.

## Background
FIELD OF THE DISCLOSURE
The present disclosure relates generally to methods for determining film thickness, and more particularly to methods for utilizing Secondary Ion Mass Spectrometry (SIMS) to determine film thickness.
BACKGROUND OF THE DISCLOSURE
As the size of semiconductor devices continues to shrink, it has been necessary for the thickness of the dielectric layers utilized in these devices to shrink as well. Consequently, the accurate measurement of the thickness of dielectric films has become an important aspect of microelectronic device development, simulation, and manufacturing.
Currently, various techniques are known to the art for measuring film thicknesses. These include Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), medium-energy ion scattering spectrometry (MEIS), Rutherford Backscattering Spectrometry (RBS), Secondary Ion Mass Spectrometry (SIMS), Auger Electron Spectroscopy (AES), and ellipsometry. However, when film thicknesses fall below about 20 Å (as is frequently the case with currently emerging technologies), the accuracy with which these thicknesses may be measured is a challenge for existing measurement techniques.
There is thus a need in the art for methods and devices for measuring film thicknesses, and especially those of thin film dielectric layers. In particular, there is a need in the art for a method for accurately measuring the thickness of dielectric films (such as SiO2 films) when the thickness of those films falls below 20 Å. These and other needs may be met by the devices and methodologies disclosed herein.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of count rate of Si and O secondary ions as a function of depth;
FIG. 2 is a flowchart of an embodiment of a process in accordance with the teachings herein;
FIG. 3 is a flowchart of an embodiment of a process in accordance with the teachings herein; and
FIG. 4 is a flowchart of an embodiment of a process in accordance with the teachings herein.



DETAILED DESCRIPTION
In one aspect, a method for measuring the thickness of a layer is provided. In accordance with the method (which is depicted in the flowchart of FIG. 2), a structure is provided 101 which comprises a first layer disposed on a second layer. The structure is impinged 103a with a first ion beam comprising a first isotope, thereby sputtering off 103b a portion of the first layer which contains a second isotope and exposing 103c a portion of the second layer. The thickness of the first layer is then determined 105 by measuring the amount of the second isotope which is sputtered off.
In another aspect, a method for measuring the thickness of a layer is provided. In accordance with the method (which is depicted in the flowchart of FIG. 3), a structure is provided 201a which includes a second layer 201b comprising a second material having the chemical composition Ya which is sandwiched between first and third layers, each comprising a first material which is essentially devoid of Y. The composition of the second layer is such 201c that a>0, and Y is an atomic species having first and second stable isotopes. The structure is impinged 203a with a first ion beam of the first stable isotope such that a portion of the first and second layers are sputtered off 203b and a portion of the third layer is exposed 203c. The thickness of the second layer is then determined 205 from the amount of the second isotope sputtered off.
In a further aspect, a method for measuring the thickness of a layer is provided. In accordance with the method (which is depicted in the flowchart of FIG. 4), a structure is provided 301a which comprises a first layer disposed on a second layer, wherein the first layer 301b comprises oxygen, and wherein the second layer comprises silicon and is essentially devoid of oxygen 301c. The structure is impinged 303a with a first ion beam comprising a first isotope, thereby sputtering off 303b a portion of the first layer which contains a second isotope and exposing 303c a portion of the second layer. The thickness of the first layer is then determined 305 by measuring the amount of the second isotope which is sputtered off.
Recently, Secondary Ion Mass Spectrometry (SIMS) has evolved in the art as a highly sensitive tool for analyzing SiO2 dielectric layer thicknesses. In this technique, a surface to be tested is bombarded with a beam of primary ions. The surface then emits secondary ions, which are identified with a mass spectrometer to provide compositional information about the surface.
SIMS may also be utilized to obtain profiles of elemental concentrations as a function of depth. Such depth profiling may be used to determine the thickness of SiO2 dielectric layers. For example, the thickness of the dielectric layer can be determined with SIMS by measuring the exact position of the dielectric layer/substrate interface during SIMS depth profiling with cesium (Cs+) primary ions. This interface is indicated by abrupt changes in the intensities of characteristic secondary ions, such as Si−, SiO− and SiO2−, when the interface is reached. These changes result from the composition changes and differences in the sputter yields and ionization probabilities attendant to compositional changes at the interface.
Although SIMS has many obvious advantages, the use of this technique for measuring very thin layers of SiO2 and other dielectrics is hindered by the matrix effect. The matrix effect arises from the fact that ionization rates of atoms are highly dependant on their chemical environment within the material being analyzed. Consequently, as the lower limits of layer thicknesses are approached and measurements are made of the signal of an ion consisting essentially of one matrix atom, this signal is generally not proportional to elemental concentrations as would be the case with thicker layers.
It has now been found that the aforementioned problem may be overcome in SIMS depth profiling by utilizing a primary ion beam comprising a first isotopic species at normal (or near normal) incidence to profile a film, and then measuring the total amount of a second isotopic species, having a different atomic weight from the first isotopic species and present in the secondary ion beam, to determine film thickness. Thus, in one preferred embodiment in accordance with the teachings herein which may be utilized to determine the thickness of an SiO2 film deposited on an Si substrate, an 18O2+ primary beam is utilized, and the total amount of 16O− in a secondary beam is measured to determine film thickness. In an alternate (but also preferred) embodiment which may be utilized for this purpose, a 16O2+ primary beam is used, and the total amount of 18O− in the secondary beam is measured to determine film thickness.
The methodologies described herein are preferably utilized to determine the thickness of a first layer comprising a first material which is disposed on a substrate (or on a second layer) comprising a second material. The first and second materials are preferably related such that the second material can be converted into the first material through exposure to an ion beam. Thus, for example, the first material may be an oxide of the second material. The isotopic species utilized in the primary ion beam is preferably selected such that, when it impinges on the second material, it converts the second material into the first material. Thus, for example, if the first material is SiO2 and the second material is Si, then the isotopic species utilized in the primary beam is preferably oxygen.
Without wishing to be bound by theory, it is believed that this approach eliminates the matrix effect, since the chemistry of the layer being exposed to SIMS analysis remains the same (due to the aforementioned conversion) as one approaches and crosses the interlayer interface. Thus, for example, in the case of an Si/SiO2/Si film structure, the Si layers are converted to SiO2 as they are impinged with the primary ion beam. As each portion of each of the Si layers is impinged, it is converted to SiO2 and is sputtered away. However, the chemistry of the SiO2 layer originally present in the film sample is unaffected by impingement of the primary ion beam. Hence, as the process advances through a sample of the film, the material being sputtered is always SiO2.
It is also believed that the use of a normal, or near normal, angle of incidence is also important in that it facilitates complete conversion of the second material into the first material, thus ensuring that the chemistry of materials sputtered off of the sample remains unchanged across the interface (and ensuring that the matrix effect is eliminated). Due to the elimination of the matrix effect, this approach may be used to measure thin films, such as dielectric films in semiconductor structures, with a higher degree of accuracy than that obtainable with conventional SIMS approaches or other methodologies currently known to the art.
The methodologies disclosed herein may be further appreciated with respect to the graph depicted in FIG. 1, which depicts the SIMS profile analysis of an SiO2 layer sandwiched between two Si layers. This graph depicts the abundance of the noted isotopic species as a function of depth. The profile was conducted using a 16O2+ primary ion beam at normal incidence.
As seen therein, the signal intensity of the secondary 30Si− ion beam remained constant across the profile, thus indicating the absence of a matrix effect (e.g., the 30Si− signal was not affected by the environment (Si layer versus SiO2 layer) that the silicon was being sampled from). On the other hand, the appearance of the 18O− signal indicates that the Si/SiO2 interface has been reached, since the use of 16O2+ in the primary ion beam ensures that only 16O− ions are sputtered off while the profile is proceeding through the Si layers. However, when the SiO2 layer is reached, the natural abundance of the 18O isotope in the dielectric layer gives rise to the 18O− signal. The 18O− signal disappears again when the SiO2/Si interface is reached.
Through the use of quantification techniques which are commonly utilized in conventional SIMS analyses, the intensity of 18O− can be converted into the atomic concentration of oxygen. These techniques typically incorporate an appropriate calibration procedure using an implant standard as described, for example, in FIG. 4 of U.S. Pat. No. 6,248,603 (Jones et al.) The total number of oxygen atoms D in a unit area (typically cm2) can then be found by integrating the atomic concentration of oxygen across the peak as show in Equation 1 below:




  
    
      
        D
        =
        
          
            ∫
            
              x
              1
            
            
              x
              2
            
          
          ⁢
          
            
              Co
              ⁡
              
                (
                x
                )
              
            
            ⁢
            
              d
              x
            
          
        
      
    
    
      
        (
        
          EQUATION
          ⁢
          
            
          
          ⁢
          1
        
        )
      
    
  




where Co(x) is the oxygen atomic concentration at depth x, x1 is the starting position of the oxygen peak, and x2 is the end position of the oxygen peak. The thickness T of the corresponding SiO2 film can then be determined from the total amount of oxygen and the atomic density of oxygen inside SiO2, which is known to be 4.6E22 atoms/cm3:





  
    
      
        
          T
          ⁡
          
            (
            cm
            )
          
        
        =
        
          D
          
            4.6
            ⁢
            E
            ⁢
            
              
            
            ⁢
            22
          
        
      
    
    
      
        (
        
          EQUATION
          ⁢
          
            
          
          ⁢
          2
        
        )
      
    
  




Note that the total dose of oxygen is here used to determine the thickness of oxide. A conventional method of measuring the thickness based on the interface between the oxide and Si would experience a large error due to a strong atomic mixing caused by bombardment of energetic primary ions resulting in significant broadening of the measured profile and associated uncertainty in determining the interface between a thin oxide and Si (see FIG. 1) It will be appreciated that, while this particular example involved an Si/SiO2/Si structure, the methodology would work equally well with an SiO2/Si structure or an Si/SiO2 structure, though the location of the 18O− signal in the profile might be different. It will also be appreciated that 18O2+ could have been used in the primary ion beam, in which case the 16O− signal could be used for integration.
Various materials may be used in the devices and methodologies taught herein. The material used in the primary ion beam is preferably a material having at least two distinct stable isotopes, since this allows the ions of the primary ion beam to be distinguished (by way of mass spectrometry) from ions in the secondary ion beam which have been sputtered off of the substrate. It is also preferred that the primary ion beam is positively charged (that is, that the primary ion beam comprises cations) and that the secondary ion beam is negatively charged (that is, that the primary ion beam comprises anions), though embodiments are possible in accordance with the teachings herein in which the opposite is true.
Moreover, when the methodology disclosed herein is utilized to determine the thickness of a first layer comprising a first material which is disposed on a second layer comprising a second material, it is preferred that the second material can be converted to the first material through exposure to the first ion beam, since this eliminates the matrix effect. Thus, for example, if the first layer comprises SiO2 and the second layer comprises Si, then a first stable isotope of oxygen may be used in the primary ion beam (this may be 16O, 17O or 18O), and the thickness of the first layer may be ascertained from the signal obtained for a secondary ion beam of a second stable isotope of oxygen (distinct from the first isotope). In particular, if a 16O2+ primary beam is utilized, then the amount of 18O− in a secondary beam may be ascertained to determine film thickness (it is less preferred to use secondary 17O− ions for this purpose, due to the strong mass interference which can occur from 16O1H−).
Likewise, if a 17O2+ primary beam is utilized, then the amount of 16O− or 18O− in a secondary beam may be ascertained to determine film thickness. Similarly, if a 18O2+ primary beam is utilized, then the amount of 16O− in a secondary beam may be ascertained to determine film thickness. Analogous embodiments are possible wherein the second layer comprises In, Ge, or SiGe, and the first layer comprises oxides or nitrides of the material of the second layer or comprises other materials chemically related to the material of the second layer.
It will be appreciated that the methodology disclosed herein is not limited to the use of oxygen isotopes in the primary beam. Thus, for example, if the layer whose thickness is to be determined is SiN which is adjacent to an Si layer or substrate, then the primary ion beam may comprise a first stable isotope of nitrogen (14N or 15N), and the total amount of the other stable isotope may be ascertained to determine film thickness. It will also be appreciated that the methodology disclosed herein is not limited to semiconductor materials. Thus, for example, if the layer whose thickness is to be determined is AlO3 which is disposed adjacent to an Al layer or substrate, then the primary ion beam may comprise a first stable isotope of oxygen (16O, 17O or 18O), and the total amount of a different stable isotope may be ascertained to determine film thickness. TABLE 1 below lists some stable isotopes that may be used in various embodiments of the methodologies disclosed herein.







TABLE 1







Stable Isotopes












Element
Stable Isotopes
Element
Stable Isotopes
Element
Stable Isotopes





Hydrogen
1H, 2H
Nickel
58Ni, 60Ni, 61Ni, 62Ni, 64Ni
Xenon
126Xe, 128Xe, 129Xe, 130Xe,







131Xe, 132Xe, 134Xe, 136Xe


Helium
3He, 4He
Copper
63Cu, 65Cu
Barium
130Ba, 132Ba, 134Ba, 135Ba,







136Ba, 137Ba, 138Ba


Lithium
6Li, 7Li
Zinc
64Zn, 66Zn, 67Zn, 68Zn, 70Zn
Cerium
138Ce, 140Ce


Boron
10B, 11B
Gallium
69Ga, 71Ga
Neodymium
142Nd, 143Nd, 145Nd, 146Nd,







148Nd


Carbon
12C, 13C
Germanium
70Ge, 72Ge, 73Ge, 74Ge
Samarium
144Sm, 150Sm, 152Sm,







154Sm


Nitrogen
14N, 15N
Selenium
74Se, 76Se, 77Se, 78Se, 80Se
Europium
151Eu, 153Eu


Oxygen
16O, 17O, 18O
Bromine
79Br, 81Br
Gadolinium
154Gd, 155Gd, 156Gd, 157Gd,







158Gd, 160Gd


Neon
20Ne, 21Ne, 22Ne
Krypton
80Kr, 82Kr, 83Kr, 84Kr, 86Kr
Dysprosium
156Dy, 158Dy, 160Dy, 161Dy,







162Dy, 163Dy, 164Dy


Magnesium
24Mg, 25Mg, 26Mg
Strontium
84Sr, 86Sr, 87Sr, 88Sr
Erbium
162Er, 163Er, 164Er, 166Er,







167Er, 168Er


Silicon
28Si, 29Si, 30Si
Zirconium
90Zr, 91Zr, 92Zr, 94Zr
Ytterbium
168Yb, 170Yb, 171Yb, 172Yb,







173Yb, 174Yb, 176Yb


Sulpher
33S, 34S, 36S
Molybdenum
92Mo, 94Mo, 95Mo, 96Mo,
Hafnium
176Hf, 177Hf, 178Hf, 179Hf,





97Mo, 98Mo

180Hf


Chlorine
35Cl, 37Cl
Ruthenium
96Ru, 98Ru, 99Ru, 100Ru,
Tungsten
182W, 183W, 184W, 186W





101Ru, 102Ru, 104Ru


Argon
36Ar, 38Ar, 40Ar
Palladium
102Pd, 104Pd, 105Pd, 106Pd,
Osmium
187Os, 188Os, 189Os, 190Os,





108Pd, 110Pd

192Os


Potassium
39K, 41K
Silver
107Ag, 109Ag
Iridium
191Ir, 193Ir


Calcium
40Ca, 42Ca, 43Ca,
Cadmium
106Cd, 108Cd, 110Cd, 111Cd,
Platinum
192Pt, 194Pt, 195Pt, 196Pt,



44Ca, 46Ca

112Cd, 114Cd

198Pt


Titanium
46Ti, 47Ti, 48Ti,
Tin
112Sn, 114Sn, 115Sn, 116Sn,
Mercury
198Hg, 199Hg, 200Hg, 201Hg,



49Ti, 50Ti

117Sn, 118Sn, 119Sn, 120Sn,

202Hg, 204Hg





122Sn, 124Sn


Chromium
52Cr, 53Cr, 54Cr
Antimony
121Sb, 123Sb
Thallium
203Tl, 205Tl


Iron
54Fe, 56Fe, 57Fe,
Tellurium
122Te, 123Te, 124Te, 126Te
Lead
206Pb, 207Pb, 208Pb



58Fe









As noted above, the angle of incidence used in the methodologies described herein are preferably normal or near-normal angles. Typically, these angles will be within ±20° of normality, preferably, these angles will be within ±10° of normality, and more preferably, these angles will be within ±5° of normality. Most preferably, a normal angle of incidence is utilized. The energy of the primary beam may vary, and may depend on the application and the ionic species of interest. Typically, the energy of the primary beam is in the range of about 100 eV to about 10,000 eV, and preferably, the energy of the primary beam is in the range of about 500 eV to about 5,000 eV
The above description of the present invention is illustrative, and is not intended to be limiting. It will thus be appreciated that various additions, substitutions and modifications may be made to the above described embodiments without departing from the scope of the present invention. Accordingly, the scope of the present invention should be construed in reference to the appended claims.

## Claims

1. A method for measuring the thickness of a layer, comprising:
providing a structure comprising a first layer disposed on a second layer;
impinging the structure with a first ion beam comprising a first isotope of an element, thereby sputtering off a portion of the first layer which contains a second isotope of the same element but different than the first isotope and exposing a portion of the second layer; and
determining the thickness of the first layer by measuring the amount of the second isotope which is sputtered off.

2. The method of claim 1, wherein the first ion beam comprises ions of element Y, wherein the first layer comprises a first material having the chemical composition XaYb wherein a, b>0, wherein the second layer comprises a second material having the chemical composition Xa, and wherein X is either an element or a compound.

3. The method of claim 2, wherein the structure further comprises a third layer including a third material having the chemical composition Xa, and wherein the first layer is disposed between the second and third layers.

4. The method of claim 2, wherein Y is selected from the group consisting of oxygen and nitrogen, and wherein X is selected from the group consisting of silicon, germanium, SiGe, aluminum, and indium.

5. The method of claim 2, wherein X is silicon, and wherein Y is oxygen.

6. The method of claim 2, wherein Xa is converted to XaYb when it is impinged with the first ion beam.

7. The method of claim 2, wherein the first ion beam comprises cations, and wherein the portion of the first layer is sputtered off as a secondary ion beam comprising anions.

8. The method of claim 7, wherein the anions are anions of Y.

9. The method of claim 1, wherein the structure is impinged with the first ion beam at an angle of incidence within the range of about 70° to about 110°.

10. The method of claim 1, wherein the first and second isotopes are distinct and are selected from the group consisting of 16O and 18O.

11. The method of claim 1, wherein the structure is impinged with the first ion beam at an angle of incidence within the range of about 85° to about 95°.

12. The method of claim 1, wherein the energy of the primary beam is within the range of about 100 eV to about 10,000 eV.

13. The method of claim 1, wherein the energy of the primary beam is within the range of about 500 eV to about 5,000 eV.

14. The method of claim 1, wherein the thickness of the first layer is no greater than 20 Å.

15. The method of claim 1, wherein the first layer is a gate dielectric.

16. The method of claim 1, wherein determining the thickness of the first layer comprises:
using mass spectroscopy to obtain a signal peak corresponding to the second isotope;
determining the atomic concentration of the second isotope from the signal peak; and
determining D, the total number of atoms of the second isotope in a unit area of the sample, from the determined atomic concentration of the second isotope.

17. The method of claim 16, where D is determined from the equation




  D
  =
  
    
      ∫
      
        x
        1
      
      
        x
        2
      
    
    ⁢
    
      
        Co
        ⁡
        
          (
          x
          )
        
      
      ⁢
      
        d
        x
      
    
  




wherein Co(x) is the atomic concentration of the second isotope at depth x, x1 is the starting position of the signal peak, and x2 is the end position of the signal peak, and wherein the thickness T of the corresponding layer is determined from the equation




  
    T
    ⁡
    
      (
      cm
      )
    
  
  =
  
    D
    d
  




wherein d is the atomic density of the second isotope within the layer.

18. A method for measuring the thickness of a layer, comprising:
providing a structure including a second layer comprising a second material having the chemical composition Yb which is sandwiched between first and third layers, each comprising a first material which is essentially devoid of Y, (wherein b>0), and wherein Y is an atomic species having first and second stable but mutually different isotopes;
impinging the structure with a first ion beam of the first stable isotope such that a portion of the first and second layers are sputtered off and a portion of the third layer is exposed; and
determining the thickness of the second layer from the amount of the second isotope sputtered off.

19. The method of claim 18, wherein the thickness of the first layer is no greater than 20 Å.

20. A method for measuring the thickness of a layer, comprising:
providing a structure comprising a first layer disposed on a second layer, wherein the first layer comprises oxygen, and wherein the second layer comprises silicon and is essentially devoid of oxygen;
impinging the structure with a first ion beam comprising a first isotope, thereby sputtering off a portion of the first layer which contains a second isotope different than the first isotope and exposing a portion of the second layer; and
determining the thickness of the first layer by measuring the amount of the second isotope which is sputtered off.