Strasbaugh Machine Description
Version 4.0 - February 1998 2 - 31
The wafer carrier mounting assembly has a flat underside with an aluminum
chuck equipped with 0 rings. The 0 rings form two vacuum chambers
between the chuck and the carrier. An applied vacuum source evacuates air
from one chamber to firmly attach the wafer carrier to the spindle drive
assembly. The end of the quill shaft is a ball joint for alignment to the center
of the wafer carrier. Vacuum for this runs between the back pressure feed
tube and the quill shaft wall. The vacuum fills the space not taken up in the
quill shaft by the back pressure feed tube, providing the vacuum source
required to attach the wafer carrier to the wafer carrier mounting assembly.
The feed tube, which supplies back pressure to the wafer during polishing,
transferring air and vacuum (as well as water for carrier flushing), is located
inside the quill shaft, running the entire length and extending out the top
above the servo motor. A rotary joint is mounted to the top of the feed tube.
The vacuum supply line attaches to the quill shaft.
To secure the wafer during polish liftoff and cleaning, controlling wafer
position and preventing breakage, the spindles use back pressure technology,
transferring air or vacuum as needed (to retain or release the wafer) down
through the carrier to the wafer. Back pressure is used for wafer transport
and unloading whether or not it is enabled for polishing.
During wafer transport to and from the table, the machine will attempt to
apply the minimum back pressure of 0 PSIA (-14.7 PSIG) of vacuum. This
condition encourages the wafer to remain on the carrier. The actual
minimum reached, shown on the status display, will be several PSIA or PSIG
below atmosphere. During unloading, back pressure is used for wafer blow
off. During idle times, the source for back pressure is set to a nominal value
near atmosphere, but this source is disconnected from the carrier. Instead,
the carrier is vented to atmosphere.
During polishing, selectable and controllable pressure is routed through the
spindle and carrier to the back side of the wafer. This pressure may be
displayed in either PSIA or PSIG (absolute pressure which is the pressure
above perfect vacuum) on the status display at the right side of the display
screen by setting calibration variable 65 at the Calibration/Configuration
Menu. If enabled at page 1 of the Machine Parameters Menu, back pressure
will be used during polishing. If not, the back side of the wafer is vented to
atmosphere during polishing. Enabling or disabling back pressure during
polishing has no effect on the other functions of back pressure, used when not
polishing.
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