SEM imaging method

1. A method of investigating a sample using Scanning Electron Microscopy, comprising the following steps:

irradiating a surface (S) of the sample using a probing electron beam in a plurality (N) of measurement sessions, each measurement session having an associated beam parameter (P) value that is chosen from a range of such values and that differs between measurement sessions;

detecting stimulated radiation emitted by the sample during each measurement session, associating a measurand (M) therewith and noting a value of the measurand for each measurement session, thus allowing compilation of a data set (D) of data pairs (P i , M i ), where 1≦i≦N;

employing a statistical Blind Source Separation technique to automatically process the data set (D) and spatially resolve the data set into a result set (R) of imaging pairs (Q k , L k ), in which an imaging quantity (Q) having a value Q k is associated with a discrete depth level L k referenced to the surface S.

2. The method according to claim 1 , wherein:

successive values of the beam parameter (P) associated with successive measurement sessions differ from one another by a substantially constant increment (ΔP); and

successive discrete depth levels in the obtained result set (R) are separated from one another by a substantially constant distance increment (ΔL).

3. The method as claimed in claim 1 , wherein:

the beam parameter (P) is selected from the group comprising beam energy, beam convergence angle and beam focal depth;

the stimulated radiation is selected from the group comprising secondary electrons, backscattered electrons and X-ray radiation; and

the measurand (M) is selected from the group comprising intensity and current.

4. The method as claimed in claim 1 , wherein the imaging quantity (Q) is selected from the group comprising intensity, current, angular distribution and energy spread.

5. The method according to claim 1 , wherein the statistical Blind Source Separation technique is selected from the group comprising Principal Component Analysis (PCA) and Independent Component Analysis (ICA).

6. The method according to claim 5 , wherein:

the statistical Blind Source Separation technique is PCA;

the beam parameter (P) comprises beam energy, whereby successive measurement sessions have a larger associated value of this parameter;

the stimulated radiation comprises secondary electrons and the measurand (M) comprises current;

the imaging quantity (Q) comprises intensity;

an alignment transform is performed on the elements of the data set D so as to laterally align and/or scale them;

an iterative series of data processing steps is performed in which, for each integral value of k in the range [2, . . . , N]:

PCA decomposition is applied to the subset of data pairs (P i , M i ), i=1, . . . , k;

an independent component of this decomposition having least correlation to said subset is identified, and the independent component is associated with level L k beneath the surface S,

whereby the result set R=((Q 1 , L 1 ), . . . , (Q N , L N )) is generated for a spectrum of discrete levels L k progressing from the surface (S) into the sample.

7. The method according to claim 6 , wherein:

components of the PCA decomposition are relatively weighted using a weight factor that, for a given component, is equal or proportional to the reciprocal of the Eigenvalue for that component; and

the result set (R) is augmented by adding to the elements of the result set (R) a factor corresponding to a matrix response of the sample.

8. The method according to claim 6 , wherein said PCA decomposition is a Karhunen-Loeve transform operation.

9. The method according to claim 1 , wherein the obtained result set (R) is post-processed using statistical noise reduction and restoration techniques.

10. The method according to claim 1 , wherein the result set (R) yields information regarding both a geometry and a material composition of the sample.

11. The method according to claim 1 , wherein:

said steps of irradiating the surface (S) of the sample, detecting stimulated radiation emitted by the sample to obtain the data set (D), and applying a statistical Blind Source Separation technique to process the data set (D), are comprised in a computational slicing step; and

said computational slicing step is combined with a physical slicing step, whereby a physical material removal method is used to physically remove a layer of material from the original surface (S), thereby revealing a newly exposed surface (S′).

12. The method as claimed in claim 11 , wherein said physical material removal method is selected from the group comprising mechanical milling with a blade device, ion milling with an ion beam, and ablation with a beam of electromagnetic energy.

13. The method as claimed in claim 11 , wherein said computational slicing step and said physical slicing step are alternately repeated in multiple iterations.

14. An apparatus constructed and arranged to carry out the method of claim 1 .

15. An apparatus for investigating a sample using Scanning Electron Microscopy, the apparatus comprising:

means for irradiating a surface (S) of the sample using a probing electron beam in a plurality (N) of measurement sessions, each measurement session having an associated beam parameter (P) value that is chosen from a range of such values and that differs between measurement sessions;

means for detecting stimulated radiation emitted by the sample during each measurement session, associating a measurand (M) therewith and noting a value of the measurand for each measurement session, thus allowing compilation of, a data set (D) of data pairs (P i , M i ), where 1≦i≦N;

means for employing a statistical Blind Source Separation technique to automatically process the data set (D) and spatially resolve the data set into a result set (R) of imaging pairs (Q k , L k ), in which an imaging quantity (Q) having a value Q k is associated with a discrete depth level L k referenced to the surface S.

16. The apparatus of claim 15 , wherein:

successive values of the beam parameter (P) associated with successive measurement sessions differ from one another by a substantially constant increment (ΔP); and

successive discrete depth levels in the obtained result set (R) are separated from one another by a substantially constant distance increment (ΔL).

17. The apparatus of claim 15 , wherein:

the beam parameter (P) is selected from the group comprising beam energy, beam convergence angle and beam focal depth;

the stimulated radiation is selected from the group comprising secondary electrons, backscattered electrons and X-ray radiation;

the measurand (M) is selected from the group comprising intensity and current;

the imaging quantity (Q) is selected from the group comprising intensity, current, angular distribution and energy spread; and

the statistical Blind Source Separation technique is selected from the group comprising Principal Component Analysis (PCA) and Independent Component Analysis (ICA).

18. The apparatus of claim 17 , wherein:

the beam parameter (P) comprises beam energy, whereby successive measurement sessions have a larger associated value of this parameter;

the stimulated radiation comprises secondary electrons;

the measurand (M) comprises current;

the imaging quantity (Q) comprises intensity;

the statistical Blind Source Separation technique is PCA;

the apparatus further comprising:

means for performing an alignment transform on the elements of the data set D so as to laterally align and/or scale them;

means for performing an iterative series of data processing steps comprising, for each integral value of k in the range [2, . . . , N]:

applying PCA decomposition to the subset of data pairs (P i , M i ), where i=1, . . . , k;

identifying an independent component of the PCA decomposition having least correlation to said subset, and

associating the independent component with level L k beneath the surface S,

means for generating the result set R=((Q 1 , L 1 ), . . . , (Q N , L n )) for a spectrum of discrete levels L k progressing from the surface (S) into the sample.

19. The apparatus of claim 1 , further comprising means for physically removing a layer of material from the original surface (S), thereby revealing a newly exposed surface (S′).

20. The apparatus of claim 19 , wherein said means for physically removing a layer of material is selected from the group comprising:

means for mechanical milling with a blade device,

means for ion milling with an ion beam, and

means for ablation with a beam of electromagnetic energy.