STRASBAUGH CONFIDENTIAL INFORMATION
2
Rough Wafers
Recently it has been attempted to grind poly-silicon wafers having small bumps. If a bump happens to lie on the
circular path taken by the probe as it travels around the wafer, the probe is smacked out of the way. It can be hit
with enough force to make it lose track of its current position. The coarse grind cycle has been modified to
recover from this loss of information.
Machine Variable 37 is called Probe Bounce Threshold. If you place a value into this variable, software will
monitor the variation in the probe reading. This monitoring takes place over the number of revolutions of the
wafer specified in machine variable 96, Grind Probe Average Revolutions. While the variation remains greater
than the Probe Bounce Threshold, the probe is said to be bouncing. During probe bouncing, the probe value
may drift farther and farther from reality, but its actual value will be ignored. Only the variation in value will be
monitored. When bouncing falls below the threshold given by this variable, the bumps will have been removed
by the grind wheel. At this point the probe will be raised and lowered to reset itself, and then the coarse grind
cycle will run to completion. If you set this variable to zero, grinding behaves normally, without examining
probe bouncing.
A second item of data is also needed in this process. Without reliable probe readings, how close can the grind
wheel approach the wafer at high speed before it risks a collision? Feed Wafer Thickness taken from the
grinding recipe will be used for this. Feed Wafer Thickness is already required to be as large or larger than the
thickest wafer. It is recommended that you add a bit more, perhaps fifty microns, as an extra cushion.
Warning. Your rough wafers may differ from the rough wafers already tested. It is strongly recommended that
you test with dummy wafers before grinding any valuable wafers.
Warning. The obstacles on some of the rough wafers tested at Strasbaugh were strong enough to bend the
probe in addition to smacking it out of the way. If your wafers damage the probe, the 7AF should not be used to
grind them until some other tool has removed the bumps.
Warning. Because of the lag in examining the variation over a large number of revolutions of the wafer, a large
value for Grind Probe Average Revolutions in variable 96 can increase the chances of overshooting your grind
target.
Other New Features
A software bug was introduced in version 5.02 that can cause a grind cycle to abort if you open the operator
door, also called the gull-wing door. That has been corrected.
The requirement that you must select Left_Off or Right_Off in machine variable 18 in order to select Special
Options in machine variable 19 has been dropped.
The following changes have been made to the display screens.
Wafer diameter and spin station vacuum and spin station rpm are added to the Home screen and to the
Inputs/Outputs screen. On the Home screen, the wafer diameter is the value from the Defaulto recipe. On the
Inputs/Outputs screen it is the wafer size that was most recently scanned. (It displays as question marks if no
wafers have been scanned.)
An indication “+TAPE” is added to the Edit Recipes screen as a reminder that the current recipe includes tape
thickness as part of its target thickness.
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