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Introduction

The alignment scheme used on the GCA steppers is very different than the one used in contact lithography.  Put simply, it is not a through the lens system - the user cannot look through the mask and lens at the pattern on the substrate.  It is a system in which all masks are aligned to the optical column, and all substrates are aligned to the optical column, but masks and substrates are not aligned directly to each other.  Consequently, both masks and wafers require alignment marks.  

Mask alignment marks are commonly referred to as fiducials.  They are placed on the mask in specific locations to allow alignment of the mask to the optical column.  Both the GCA PG 3600F and the DWL 66 can pattern fiducials on the mask without including them in your CAD data.

The first level mask in a multilayer process must include special wafer alignment marks--sometimes called keys--which match marks built into the stepper alignment microscope.  Subsequent levels of lithography do not need alignment marks, unlike the case for contact lithography.  Although this may seem awkward, it actually produces very good alignment, typically ± 0.25 µm on the GCA 6300s aligning manually, and ± 0.15 µm on the GCA AutoStep 200 using automated alignment.  There are special considerations in placing your alignment marks.  Please consult a staff member before making your masks.

The fiducials and wafer alignment marks are different for the GCA 6300s and AutoStep 200 systems.  For the sake of clarity they are covered separately. 

GCA 6300 5x and 10x

Mask Alignment Marks (Fiducials)

Masks made for the GCA 6300 5x and 10x use a set of simple fiducial marks and a manual microscope-based system to align the mask to the optical column.  All adjustments to the position of the mask are made by the operator of the tool.  The GCA 3600F PG and the DWL 66 mask writers can automatically pattern these marks onto a mask while it is being writted without including it your the pattern data.  The fiducial marks are composed of four boxes where the Cr has been removed creating a dark Cr cross.  Sets of crosses are placed 103 mm apart on the mask as shown in the figure below

 

Global Alignment

Global alignment is performed using a set of global alignment marks.  The global alignment marks for the GCA 6300 steppers has a specific pattern shown below. 

 

 

To perform global alignment, two of these marks are required.  This allows the user of the stepper to adjust for X and Y axis offsets as well as rotation.  The GCA 6300 steppers have a split field video microscope that allows the user to view a set of alignment marks.  The objectives of this microscope are placed at a fixed distance of 63.5 mm.  This means that some amount of planning must be performed before the masks are made to account for

 

 

 

Planning out your CAD such that two global alignment marks are spaced 63.5 mm apart is not always possible

 

 

 

 

 

 

 

 

Die by Die Alignment

 

GCA Autostep 200

Mask Alignment Marks (Fiducials)

The standard reticle phase and alignment window is negative. A negative phase indicates that the alignment window is a clear opening in the chrome background of the reticle as shown below. Positive phase reticles exist, but are not commonly used. The RMS is capable of using either positive or negative phase reticles. The position of the alignment window corresponds to RMS aligner detectors. The detector observes the position of the window with respect to the aligner fiducial.

 

 

INSITU Marks

To ensure optimum overlay, it is important that the alignment of the product reticle in relation to the previous alignment of the calibration reticle be determined accurately. In order to achieve this, upon completion of the reticle loading and the alignment by the RMS, the alignment strategy uses INSITU to accurately determine the product reticle position by aligning to the INSITU reticle windows located at fixed positions on the product reticle.

The location and dimensions of the INSITU reticle windows should comply with the figures below.

 

 

 

Global Alignment

By aligning 2 global alignment marks on either side of the wafer for X, Y and theta, total overlay may be achieved for less critical alignments as shown in the figure below:

 

The global alignment target is used to align the entire wafer during the global alignment process. The marks located at 45 and 135 degree are used for the automatic global alignment performed by the AWA/D while the 90 and 180 degree marks are typically used during manual operator alignment. The global alignment target shown below is a typical target used for either manual or automatic wafer global alignment.

 

Die by Die Alignment

 

 

 

 

 



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