ZMAP Product Family
ZMAP consists of both hardware (ZMAP reticle) and software.
ZMAP has four different modes of focus measurements that allows the user to measure the focus components of a lithography exposure tool. This helps the user to understand the contribution from each item and make better adjustments using the data.
4 Modes of ZMAP Operation:
- Raw mode: This measures all the contributions together as in other industry techniques.
- Static Lens Mode: This mode allows users to measure the static lens contribution of focus with wafer contribution, some reticle shape, and stage contribution removed.
- Dynamic Lens Mode: This mode measures the dynamic lens contribution of focus with wafer contribution, some reticle shape, and stage contribution removed.
- Dynamic Scan Mode: This mode measures the repeatable part of Z and Roll variations in scanning direction due to the scan synchronization error between reticle and wafer stages with wafer contribution, some reticle shape and stage contribution removed.
In the lithography exposure tool, the focus contains the following contributions in both static and dynamic exposure modes:
- Wafer height (both wafer flatness & resist thickness variations)
- Reticle flatness
- Stage (Simply piston and tilt for static exposure mode and scan synchronization error between reticle and wafer stages for dynamic mode)
- Lens (Z4 of Zernike coefficients)
There are industy standard techniques to measure the focus of the exposure tools, but the focus values that those techniques measure contain all the contributions listed above together.
Components of Focus in Lithography
Wafer Height (FW):
- This component consists of wafer flaness variation.
- This component happens in both static and dynamic exposure modes.
- As wafer flatness varies, the focus values due to this effect (Focus Wafer = FW) would vary as well (shown in the diagram below).
Reticle Flatness (FR):
- This component comes from the variation of the flatness of the bottom side of the reticle.
- As shown in the diagram, the variations in the distance from bottom surface of the reticle to the reference point below. This would show up as variations in focus (FR).
- This component happens in both static and dynamic exposure modes.
Stage in Static Exposure Modes (FSS):
- Stage contribution on the focus in the static exposure mode is a combination of piston (motion in Z, shown in diagram B and tilt (in both Xand Y, shown in diagram C).
- This contribution could be cuased by the wafer stage installation and/or debris on the wafer stage that vary the focus.
Stage in Dynamic Exposure Modes (FSD):
- This component occurs due to the scan synchronization error between reticle and wafer stages. The component is in terms of piston (Z motion variation in scanning direction, Y), roll (X tilt variation in scanning direction Y) and pitch (X tilt variation in scanning direction, Y).
- The averaged pitch (moving average over slot height) in scanning direction would show up as piston in Y direction.
Lens (FL):
- Focus variation from lens is due to Z4 term of Zernike Coefficients of lens aberration.
- If the lens focus is out of focus in either positive or negative direction from the best focus (red line in the diagram below), the focal point would be either top or bottom of the best focus (purple and green dotted lines).
- In static exposure mode, the lens focus varies across the exposed region.
- In dynamic mode, the lens focus varies in X direction (across the slit), but it is the same in the Y direction (scanning direction).
