Methods of cross-section imaging of an inspection volume in a wafer
The present disclosure relates to dual beam device and three-dimensional circuit pattern inspection techniques by cross sectioning of inspection volumes with large depth extension exceeding 1 μm below the surface of a semiconductor wafer, as well as methods, computer program products and apparatuses for generating 3D volume image data of a deep inspection volume inside a wafer without removal of a sample from the wafer. The disclosure further relates to 3D volume image generation and cross section image alignment methods utilizing a dual beam device for three-dimensional circuit pattern inspection.
1 . A method, comprising:
alternately imaging and milling into an inspection volume of a wafer at a slant angle to obtain a sequence of N cross-section surfaces of the wafer and a corresponding sequence of N cross-section image slices of the wafer;
determining a first orientation direction of first semiconductor features of the wafer, the first semiconductor features forming a first plurality of first cross-section image features in the sequence of N cross-section image slices of the wafer; and
computing a virtual cross-section image perpendicular to the first orientation direction, the virtual cross-section image comprising a plurality of virtual cross-section image pixels,
wherein, for each virtual cross-section image pixel, the method comprises:
computing a pixel value by projecting a subset of at least one cross-section image slice of the sequence of N cross-section image slices in the first orientation direction;
selecting the subset of the at least one cross-section image slice by evaluating a distance of each of the sequence of N cross-section image slices to the virtual cross-section image pixel in the first orientation direction; and
selecting a first cross-section image slice having the shortest distance to the virtual cross-section image pixel in the first orientation direction.
2 . The method of claim 1 , further comprising selecting a second cross-section image slice of the subset of the at least one cross-section image slice as the cross-section image slice having the second shortest distance to the virtual cross-section image pixel in the first orientation direction.
3 . The method of claim 1 , further comprising selecting further cross-section image slices of the subset of the at least one cross-section image slice in sequence of increasing distances to the virtual cross-section image pixel in the first orientation direction.
4 . The method of claim 1 , wherein the projection of the subset of the at least one cross-section image slice comprises an interpolation of at least a subset of the first plurality of first cross-section image features.
5 . The method of claim 4 , further comprising combining the projection and the interpolation with at least one member selected from the group consisting of a feature extraction, a thresholding operation, a contour interpolation, and a model-based interpolation.
6 . The method of claim 1 , further comprising, for each of the sequence of N cross-section image slices, generating a depth map.
7 . The method of claim 6 , further comprising, for each depth map, generating the depth map from a plurality of second cross-section image features, wherein the plurality of second cross-section image features represent cross sections through second semiconductor features oriented in a second orientation direction perpendicular to the first orientation direction.
8 . The method of claim 7 , wherein each of the second semiconductor features comprise at least one member selected from the group consisting of an isolator line or layer, a metal line or layer, and a semiconductor line or layer of an integrated semiconductor circuit inside the inspection volume of the wafer.
9 . The method of claim 7 , further comprising, in each of the sequence of N cross-section image slices, determining:
at least one of the first cross-section image features; and
at least one of the second cross-section image features.
10 . The method of claim 7 , further comprising, for each depth map, generating the depth map by determining a depth of a first cross-section image feature in a cross-section image slice from lateral positions of at least two of the second cross-section image features.
11 . The method of claim 1 , wherein each of the first semiconductor features comprises at least of one member selected from the group consisting of a via, a high aspect ratio (HAR) structure, and a HAR channel of an integrated semiconductor circuit inside the inspection volume of the wafer.
12 . The method of claim 1 , wherein the first orientation direction is in a direction perpendicular to the wafer surface, and the virtual cross-section image slice is computed in a plane parallel to the wafer surface at a depth below the wafer surface.
13 . The method of claim 12 , further comprising, for each virtual cross-section image pixel coordinate, determining the subset of the at least one cross-section image slices by selecting the cross-section image slice with a minimum distance to the depth below the wafer surface.
14 . The method of claim 13 , further comprising selecting at least two additional cross-section image slices of the subset of the at least one cross-section image slice in sequence of increasing distances to the virtual cross-section image pixel in the direction perpendicular to the wafer surface.
15 . The method of claim 12 , further comprising adjusting the depth direction perpendicular to the wafer surface according to the depth of layers parallel to the wafer surface, wherein the layers are formed by second semiconductor features oriented in a second orientation direction parallel to the wafer surface.
16 . The method of claim 12 , further comprising computing the virtual cross-section image slice at a depth direction perpendicular to the wafer surface inside an isolation layer between two adjacent metal layers or word-lines.
17 . The method of claim 12 , further comprising computing the virtual cross-section image slice is computed at a depth direction perpendicular to the wafer surface inside a metal layer or inside a word-line.
18 . The method of claim 1 , further comprising orienting an optical axis of a charged particle beam imaging system configured to acquire the sequence of N cross-section image slices such that an angle between the optical axis of the charged particle beam imaging system and a normal to the wafer surface is 0°.
19 . The method of claim 1 , further comprising orienting an optical axis of a charged particle beam imaging system configured to acquire the sequence of N cross-section image slices perpendicular such that the optical axis of the charged particle beam is perpendicular an optical axis of a focused ion beam used to mill into the inspection volume.
20 . The method of claim 1 , further comprising performing a mutual lateral alignment of each of the sequence of N cross-section image slices with at least one common cross-section image feature.
21 . The method of claim 20 , comprising further forming at least one alignment feature in proximity of the inspection volume configured to form the at least one common cross-section image feature to perform the mutual lateral alignment.
22 . The method of claim 20 , wherein performing the mutual lateral image alignment comprises subtraction of an image distortion deviation.
23 . The method of claim 1 , wherein the number N is at least 10.
24 . The method of claim 1 , further comprising:
loading the wafer on a wafer support table in a dual beam device, the dual beam device comprising a focused ion beam (FIB) column and a charged particle imaging device, an optical axis of the FIB column being at the slant angle GF with a surface of the wafer support table, the charged particle imaging device having an optical axis that forms an intersection point with the optical axis of the FIB column; and
moving the wafer support table to bring a first measurement site on the wafer in coincidence with the intersection point.
25 . The method of claim 24 , wherein, when of obtaining the sequence of N cross-section image slices in the inspection volume, the wafer is not moved.
26 . A system comprising:
one or more processing devices; and
one or more machine-readable hardware storage devices comprising instructions that are executable by the one or more processing devices to perform operations comprising the method of claim 1 .
27 . The system of claim 26 , further comprising:
a wafer stage having a wafer support table configured to hold the wafer;
a focused ion beam (FIB) column configured to mill and expose the sequence of N cross-section surface in the inspection volume; and
a charged particle imaging device configured to image the sequence of N cross-section surfaces to form the sequence of N cross-section image slices,
wherein:
the focused ion beam comprises an optical axis forming the slant angle with a surface of the wafer support table; and
the charged particle imaging device comprises an optical axis that forms an intersection point with the optical axis of the FIB column.
28 . The system of claim 27 , wherein the slant angle is between 20° and 60°.
29 . The system of claim 27 , wherein the optical axis of the charged particle imaging device is perpendicular to the wafer surface.
30 . The system of claim 27 , wherein the charged particle imaging device is perpendicular to the optical axis of the FIB column.
31 . One or more machine-readable hardware storage devices comprising instructions that are executable by one or more processing devices to perform operations comprising the method of claim 1 .