This document describes how to create the alignment marks on your mask for multi-level exposures on the Nikon stepper. You will need to create your own configuration file on the Nikon, so it knows where to find your alignment marks and where to expose on the wafer. Most of the file-creation procedure is described in the Nikon System Operator Training Manual and the instruction sheet attached to the Nikon.

The first exposure has nothing to align to, so there's not much to say about it. Just make sure that you have a reticle and procedure in the file with an exposure number of 1.

The second (and later) exposures align to the marks from the first exposure. Thus, you will need to create a reticle and procedure in the file with an exposure number of 2 (or higher) and include information on where the alignment marks were on the first exposure.

You should start by using SNFMAN as your APF (which describes how the Nikon searches for the alignment marks).

The Nikon stepper requires several different types of alignment targets, or marks, to be present in each exposure field. There are three methods of alignment available for the NSR: Global Alignment, Step Alignment and Enhanced Global Alignment. Each type of alignment requires a different type of mark. In addition to wafer and exposure field alignments, the Nikon has several advanced alignment measurement capabilities. For more information, see the Nikon manual and the recommendations at the end of this section.

The Nikon always requires at least the Wafer alignment marks and the Laser Step Alignment marks to be present for a multi-level exposure. The Step alignment marks may be added to ensure a fail-safe in case of laser alignment problems and for runout measurement and correction.

The reticle also contains a set of reticle alignment marks (RX and RY) outside the exposure field. These marks are used to align the reticle (mask) to the machine.

The per-exposure Nikon alignment marks may be clear or dark (trench or plateau) structures. The Reticle alignment marks must be dark (i.e. the plus and minus must be chrome and the area surrounding them must be clear).

A design file of alignment marks in Tanner L-Edit format is available for download: Nikon_marks.tdb

A design file of alignment marks in GDS-II format is available for download: Nikon_marks.gds

Note: all dimensions given below are: in microns, on-wafer dimensions, and on-center spacings unless otherwise noted.

For Wafer Alignment, there are three Procedure Types to select from:

PROCEDURE TYPE XA	PROCEDURE TYPE XS	PROCEDURE TYPE XF
1. Global alignment	1. Global Alignment	1. Global Alignment
2. Step Alignment test	2. Step Alignment	2. Enhanced Global Alignment
3. Runout measurement

Use the above table to decide which marks to include in your layout. For example, if runout measurement is important then the Sx and Sy marks must be included in the mask layout. Procedure Type XS is typically used at SNF.

These four marks align the wafer to the wafer microscopes. Although these marks are included in every exposure field, only four of them are used by the Nikon:

You will get best results by ensuring good rotational alignment. This is the first part of the alignment process, where it tries to find WY and Wtheta. If the X alignment is off by a small amount, it can be corrected via the Laser Step Alignment marks.

The Nikon manual calls for W-marks to consist of 2um x 6.5um rectangles, set at 45 degrees, spaced 5.6um apart, in 3 - 300um rows. However, SNF users have been using smaller rectangles that are aligned to the x-y axis of the wafer because they survive repeated growth/deposition steps better. The alignment accuracy is the same for either mark style. For these marks, the rectangles in the WX marks are 4um x 3um, spaced 6um apart in vertical rows, with the row length at least 300um (i.e., 50 or more rectangles). The other W-marks are the opposite: 3um x 4um rectangles, spaced 6um apart horizontally, at least 300um long. The auto-search function of the Nikon uses a pattern of three rows of marks, with different spacing between the rows. For the WX marks, the space between the left and the center rows is 20um, while the space between the right and center rows is 26um. For the other marks, the space between the top and the center rows is 20um, while the space between the bottom and center rows is 26um. See the examples in Figure 1.

The W-marks are included in every exposure field, but four of them must be designated for the Nikon to use to align the wafer. These four must be located within specific regions on the wafer, so the user's exposure pattern must accomodate this requirement.

The location of the W-marks is shown in Figure 1. WY and Wtheta must be selected so that they are exactly 63.5mm apart on the wafer surface at the same y-coordinate (i.e. make sure you account for the die pitch, including the gap between dice). WR must be to the right of the Wtheta alignment mark, also at the same y-coordinate. WX must be in the right half of the wafer. Note that Figure 2 shows the approximate regions allowable for the W marks. See the Nikon Operators manual, page 5-13, for precise boundaries.

When you create a procedure file for your mask, you will need to specify the X and Y coordinates of the WY, WR, and WX marks. The Wtheta mark is calculated to be 63.5 mm in the +X direction from the WY mark. You should use the center of the middle row of rectangles as the coordinates for these marks. See the small black plus in Figure 1c.
When programming the procedure file, you will need to specify the column and row locations of each mark. Keep in mind how you intend to place your dice on the wafer when picking locations for your W marks.

LSA marks are used to perform step alignment and other step and pattern alignment corrections. Step alignment is the process of measuring and compensation offsets unique to each exposure field on the wafer during wafer exposure.

The LSA marks are 7x7 arrays of 4x4um squares. The array is spaced at 20um x 8um. The LSAX mark array spacing is horizontal (20um in x, 8um in y), and the LSAY mark is vertical (20um in y, 8um in x). See Figure 3.

Each exposure field should include two LSA marks: LSAX and LSAY. They can be placed anywhere in the exposure field.

When you create a procedure file for your mask, you will need to specify the X and Y coordinates of the LSAX and LSAY marks. You should use the center of the center square as the coordinates for these marks. See the small black plus in Figure 3b.

Step alignment marks (SX and SY) are used for Enhanced Global Alignment (EGA). EGA is an alternative when regular global and step alignment cannot be executed. For EGA, LSAX and LSAY, SX and SY marks must be present and the location of the reticle windows must be known. S marks can also be used for runout, orthogonality, and other advanced step alignment procedures. EGA and S marks are typically not used at SNF. For more information, see the Nikon manual.

SNF users are advised to design their masks for an XS procedure, described in the table above. This procedure allows good layer-to-layer step alignment with minimal complexity of layout and programming.

The XS procedure requires each exposure field to have two types of alignment marks: W-marks and LSA-marks. Step alignment marks are not required.

The SNF mask shop or outside vendors can put the correct reticle marks on your mask. Do not worry about including them in your design file.

0-degree rectangular W marks are recommended, rather than 45-degree marks.

Updated JAN2003 MW

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Last Modified 12/02/2003


Figure 1a WX mark (diagonal style) (photograph)	Figure 1b WY mark (diagonal style) (photograph)	Figure 1c WX mark (straight style) (layout)


Figure 3a LSAX mark (photograph)	Figure 3b LSAX mark (straight style) (layout)	Figure 3c LSAY mark (photograph)

NIKON Step-and-Repeat Exposure System (NSR)