Operation and Maintenance Manual Model 7AA Wafer Grinder
7 - 18 Version 1.2 - April 18, 1996
The precision calibration switch is activated by a small plunger
extending out the bottom of the switch. This plunger has a travel
length of approximately 1/4". Within this travel the switch will
change state at a precise point. In order to prevent false switch
signals the machine should depress the plunger a certain distance
before tripping the switch. This distance is defined as "Spindle
Pretravel".
Spindle Pretravel is a machine parameter that is used during
machine initialization. Without this compensation a wafer would be
ground too thin after initialization (therefore it is always a positive
value to increase the wafer thickness). Spindle Pretravel
compensation provides two important functions: 1) to provide
thickness accuracy for wafers ground immediately after machine
initialization (minimizes error compensation and 2) to accurately
calibrate coarse wheel position for optimizing dual grind cycle
performance. It represents the amount of travel of the spindle that
corresponds to the distance moved before tripping the switch. As
mentioned above a safety margin is desired to prevent false switch
signals caused by grind spindle motion. Also, the larger the
pretravel required for initialization the longer a force grinding
condition will exist before the switch changes state. For example, if
a pretravel value of 100 microns is required for machine
initialization then a force grinding condition will be detected
approximately 100 microns after it begins. Undetected force
grinding conditions will have a negative effect on wafer thickness
control. Therefore it is also desirable to maintain a low pretravel
value to detect force grinding conditions as soon as possible.
A desirable range for pretravel is 45-70 microns. It is best to use
the micrometer adjustment to obtain a pretravel value in this range
for initialization. Screwing the micrometer head up toward the
calibration switch will increase the required pretravel value.
To determine spindle pretravel calibrate the machine and perform
the test before any error or wheel wear compensation is done (this
is true if the "Actual" and "Offset" coordinates are the same for
both wheels). Spindle pretravel is determined by performing a
single grind cycle with the coarse wheel using the exact grind
parameters as will be used for the coarse wheel during production
grinding. If a single grind is required for production then the same
cycle should be used for determining spindle pretravel. If a dual
grind is required then a single grind program should be used with
all parameters identical to those for the coarse wheel portion of the
dual grind cycle.
To Next Page
Loading...