The "Cross_marks" file contains a set of alignment marks suggested for use in the contact aligners. These are only suggestions; many other types of marks can be used. The Cross Marks file is provided in Tanner L-Edit (version 7) and GDS formats.
The Cross Marks are a series of four crosses which decrease in size. The crosses overlap slightly with the cross on the previous layer, and this overlap can be seen in the microscopes in the contact aligner. By aligning the larger crosses first, and proceeding to the smaller patterns, you can achieve good alignment between layers.
The file contains four basic cells: bases and marks, for dark and clear data layers. Marks on the mask align to bases already etched on the wafer by the previous mask.
Example: your first mask level (mask 1) patterns some trenches in an oxide layer (clear mask). The next mask level (mask 2), patterns some metal lines (dark mask), and should be aligned to mask 1. The third mask level (mask 3) opens contacts to the metal (clear layer), and should be aligned to mask 2.
The cross marks required for these masks are:
- Mask 1: "base for dark"
- Mask 2: "dark mark" and "base for clear"
- Mask 3: "clear mark"
The alignment marks for this example are included in the Cross Marks file as "align_marks_example_1".
The Cross_Marks file also contains cells that represent the dimensions of a typical 4 inch (100mm) wafer. These cells can be used to help you position your alignment marks correctly on your wafer. The cells include:
- "4in_wafer" : the outline of a wafer with a single primary flat
- "4in_ebr_2mm_5mm" : the outline of the 2mm and 5mm photoresist edge bead removal steps
- "4in_frontside_align_min" : approximate region at the center of the wafer where the frontside optics of the contact aligners (evalign and karlsuss) CANNOT reach
- "4in_evalign_backside" : approximate regions where the backside optics of the evalign CAN reach
- "4in_karlsuss_backside" : approximate location of the vacuum lines for the standard karlsuss chuck (these can obscure backside alignment marks)
Frontside alignment marks should not be placed inside the circle indicated in the cell "4in_frontside_align_min", because the frontside optics cannot move to the center of the wafer. For backside alignment using the evalign, marks should be placed inside the rectangles indicated by the cell "4in_evalign_backside". For backside alignment using the karlsuss, marks should be placed away from the vacuum lines, or a different chuck should be used. See the documentation for the aligners for more information.
WARNING: Be sure you understand any alignment mark scheme thoroughly before you submit your masks! SNF is not responsible for making new masks with corrected alignment marks.
This information has been compiled and presented by the author as a public service. Please note that all the information contained in the accompanying files is approximate and incomplete. Neither the author, nor SNF, nor Stanford University, accepts any responsibility for damages arising from the use of the information presented.
-MH MAR2001, updated MH NOV2003