Method and apparatus for detecting defects in wafers

• US 6,091,249 A
• Filed: 01/23/1998
• Issued: 07/18/2000
• Est. Priority Date: 01/13/1997
• Status: Expired due to Term

• Current Assignee: Applied Materials Incorporated
• Original Assignee: Schlumberger Technologies Incorporated (Schlumberger NV)
• Inventors: Talbot, Christopher Graham, Orjuela, Luis Camilo, Wang, Li, Lo, Chiwoei Wayne
• Primary Examiner(s): BROWN, GLENN WARNER
• Application Number:
  US09/012,277
• Time in Patent Office: 907 Days
• Field of Search: 324/452, 324/456, 324/501, 324/537, 324/750, 324/751, 324/754, 324/765
• US Class Current: 324/754.22
• CPC Class Codes: G01R 31/307 of integrated circuits

Abstract

A method for detecting electrical defects in a semiconductor wafer, includes the steps of: a) applying charge to the wafer such that electrically isolated structures are raised to a voltage relative to electrically grounded structures; b) obtaining voltage contrast data for at least a portion of the wafer containing such structures using an electron beam; and c) analyzing the voltage contrast data to detect structures at voltages different from predetermined voltages for such structures. Voltage contrast data can take one of a number of forms. In a simple form, data for a number of positions on a line scan of an electron beam can be taken and displayed or stored as a series of voltage levels and scan positions. Alternatively, the data from a series of scans can be displayed as a voltage contrast image. Analysis can be achieved by comparison of one set of voltage contrast data, for example voltage contrast data from one die on a wafer, with one or more other such sets, for example voltage contrast data for corresponding structures on one or more preceding dice, so as to determine differences therebetween.

Claims

1. A method of detecting electrical defects in a die on a semiconductor wafer, comprising:

a) applying charge to a predetermined region of the wafer such that electrically isolated structures in the predetermined region are raised to a voltage relative to electrically grounded structures;

b) probing a portion of the predetermined region so as to obtain voltage contrast data for the structures in the portion of the predetermined region; and

c) analyzing the voltage contrast data to detect structures at voltages different from reference voltages that would be anticipated for such structures if such a charge were applied and the structures were non-defective, thereby identifying defective structures;

wherein the step of applying the charge is performed so as to apply charge to the predetermined region at a significantly lower resolution than the resolution at which the region is probed.

2. A method as claimed in claim 1, wherein step a) comprises flooding the predetermined region with relatively low energy electrons.

3. A method as claimed in claim 2 wherein the flooding step applies electrons to at least a major part of the predetermined region in a single step.

4. A method as claimed in claim 1, wherein step b) comprises scanning a charged particle beam across the predetermined region in a series of scan lines which intersect the structures.

5. A method as claimed in claim 4, wherein the charged particle beam scans substantially less than the whole area of the predetermined region.

6. A method as claimed in claim 1, wherein step b) comprises obtaining a voltage contrast image of the portion of the region.

7. A method as claimed in claim 6, wherein step c) comprises comparing the voltage contrast image to an image of corresponding structures at the reference voltages.

8. A method as claimed in claim 6, wherein step c) comprises comparing the voltage contrast image with an image of corresponding structures elsewhere on the wafer and determining any differences between the images.

9. A method as claimed in claim 1, wherein the reference voltages are derived from design data for the die and step c) comprises comparing the voltage contrast data with the reference voltages.

10. A method as claimed in claim 1, wherein step c) comprises analysis of the voltage contrast data to determine the presence of a defect selected from the group consisting of shorts, gate shorts, open vias, broken conductor lines, bridging conductors and missing diffusion.

11. A method as claimed in claim 1, further comprising optically inspecting the wafer for defects and comparing the defects detected by such inspection with defects detected by analysis of the voltage contrast data.

12. A method as claimed in claim 11, further comprising determining which defects detected by optical inspection have no electrical effect on the structures by comparison with the voltage contrast data.

13. A method as claimed in claim 1, comprising repetitions of steps a)-c) separated by manufacturing process steps applied to the wafer.

14. A method as claimed in claim 1, wherein the predetermined region to which the charge is applied includes a conductor which has an unwanted resistive short to another portion of the die, and wherein the amount of charge applied is sufficient to charge up said conductor to establish voltage contrast between floating and grounded conductors.

15. A method as claimed in claim 1, wherein the predetermined region to which the charge is applied includes a large floating-conductor network, and wherein the amount of charge applied is sufficient to charge the network to establish voltage contrast between the network and surrounding structure of the die.

16. A method as claimed in claim 1, wherein the die comprises a plurality of large networks, wherein the predetermined region to which the charge is applied comprises a portion of a first one of the networks, and wherein analyzing the voltage contrast data comprises determining whether charge is detectable on a second on e of the networks.

17. Apparatus for detecting electrical defects in a die on a semiconductor wafer, comprising:

a) means for applying charge to a predetermined region of the wafer such that electrically isolated structures in the die are raised to a voltage relative to electrically grounded structures;

b) a probe, having significantly higher resolution than the means for applying charge, for obtaining voltage contrast data for a portion of the predetermined region containing such structures;

c) means for determining reference voltages for such structures, should they be so charged and should they be non-defective;

d) means for analyzing the voltage contrast data to detect structures at voltages different from the reference voltages for such structures, thus determining which structures are defective.

18. Apparatus as claimed in claim 17, wherein the means for analyzing voltage contrast data compares the voltage contrast data taken from the portion of the predetermined region with voltage contrast data obtained from a corresponding region of another device.

19. Apparatus as claimed in claim 17, wherein the means for determining reference voltages comprises means for using design data for the die to determine the reference voltages and means for analyzing the voltage contrast data comprises means for comparing the voltage contrast data with the design data reference voltages.

20. Apparatus as claimed in claim 17, wherein the probe comprises an electron beam probe arrangement which scans an electron beam across the die.

21. Apparatus as claimed in claim 20, wherein the electron beam probe also includes a detector for secondary electrons emitted from the die as the electron beam is scanned across the die.

22. Apparatus as claimed in claims 20, wherein the electron beam probe scans the beam across substantially less than the whole area of the predetermined region.

23. Apparatus as claimed in claim 17, wherein the means for applying the charge to the wafer comprises a charged particle flood gun.

24. Apparatus as claimed in claim 23, wherein the charged particle flood gun applies relatively low energy electrons to the wafer.

25. Apparatus as claimed in claim 23, wherein the probe comprises a charged-particle-beam column having a column axis and an objective lens, and wherein the charged particle flood gun is positioned within the column so that the flood beam is parallel to the column axis when the flood beam exits the objective lens.

26. Apparatus as claimed in claim 25, further comprising a controllable deflector for selecting whether the primary beam or the flood beam is applied to the wafer.

27. Apparatus as claimed in claim 25, wherein the column comprises a set of raster-scanning coils for for scanning the flood beam over a region of wafer significantly larger than the flood-beam spot for scanning the primary beam over a region of the wafer larger than the primary-beam spot.

28. A method of detecting electrical defects in a device on a semiconductor wafer, the device having some structures at ground voltage and other structures at a floating voltage relative to ground, the method comprising:

a) scanning a charged particle beam across the surface of a predetermined region of the device in a series of spaced scan lines so as to intersect the structures;

b) obtaining voltage contrast data for each scan line in the series;

c) analyzing the voltage contrast data from the scan lines to determine the presence of a structure at a different voltage from a reference voltage anticipated for that structure if that structure were so charged and non-defective, thus identifying defective structures.

29. A method as claimed in claim 28, wherein the charged particle beam comprises an electron beam which is scanned at a substantially greater resolution along the scan lines compared to the spacing of the scan lines.

30. A method as claimed in claim 29, wherein the electron beam is scanned so as to cover less than 50% of the predetermined region.

31. A method as claimed in claim 29, wherein the spacing of the scan lines is selected so as to ensure that substantially all of the structures are intersected by the scan lines.

32. A method as claimed in claim 28, comprising comparing the voltage contrast data for a scan line with that obtained for a corresponding line on another device on the wafer.

33. Apparatus for detecting electrical defects in a device on a semiconductor wafer, having some structures at ground voltage and other structures at a floating voltage relative to grounds comprising:

a) a charged particle beam probe for scanning a charged particle beam across a predetermined region of a surface of the device in a series of spaced scan lines so as to intersect structures in the device;

b) a secondary particle detector for obtaining voltage contrast data for the scan lines;

c) means for analyzing the voltage contrast data to determine the presence of a structure at a different voltage from a reference voltage for that structure should the structure be so charged and non-defective, thus identifying defective structures.

34. Apparatus as claimed in claim 33, wherein the charged particle beam probe comprises an electron beam which is scanned at a substantially greater resolution along the scan lines compared to the spacing of the scan lines.

35. Apparatus as claimed in claim 34, wherein the probe scans the electron beam over less than 50% of the predetermined region.

36. Apparatus as claimed in claim 33, wherein the means for analyzing the voltage contrast data comprises means for comparing the data with corresponding data obtained from another device so as to determine any difference there between.

37. Apparatus as claimed in claim 33, further comprising a stage for the wafer which is capable of moving the wafer relative to the electron beam probe.

38. Apparatus as claimed in claim 37, wherein the column comprises electrostatic deflectors for scanning the flood beam over a region of the wafer significantly larger than the flood-beam spot.

39. Apparatus as claimed in claim 37, wherein the flood gun is capable of delivering a flood beam having a beam current of several hundreds of microphones.

40. Apparatus as claimed in claim 37, wherein the column comprises raster-scanning coils for scanning the flood beam over a region of the wafer significantly larger than the flood-beam spot.

41. Apparatus as claimed in claim 37, wherein the probe further comprises a primary-beam source, the apparatus further comprising beam-switching elements for alternately applying to the wafer a primary beam from the source and a flood beam from the flood gun, without changing working distance of the column relative to the wafer.

42. Apparatus as claimed in claim 41, wherein the deflector comprises a pair of substantially concentric partial spherical electrodes.