Wafer support with electrostatic discharge bus

Details Wafer support with electrostatic discharge bus Wafer support with electrostatic discharge bus

1999-06-21

2001-05-29

Fleming, Fritz (Department: 2836)

Electricity: electrical systems and devices

Discharging or preventing accumulation of electric charge

C361S220000

Reexamination Certificate

active

06239963

ABSTRACT:

TECHNICAL FIELD
The invention relates to wafer handling in semiconductor processing, and in particular, to electrostatic discharge of wafers during processing operations.
BACKGROUND ART
In semiconductor manufacturing, operations wafers and their carriers are handled by robots and manufacturing process equipment. This creates static charges on the wafers and carriers. These charges can cause wafer damage through arcing to ground and contamination when airborne particles are attracted to the wafer. Wafers are held in cassettes which need to be changed when moving to or from a process environment. To change cassettes, wafers are lifted out of one cassette by comb like fingers made of insulative material which project into a cassette from below, and are then lowered into a new cassette which replaces the old cassette.
In the process of lifting wafers out of a first cassette, the wafers become highly electrically charged when extricated from the cassette because of decreasing capacitance relative to ground paths. If this charge is not removed, highly charged wafers will be placed in the new cassette and enter a new process environment in a condition where damage to the wafer could occur by sparking or arcing.
Many times manufacturers of wafer and wafer carrier handling equipment use static dissipative plastic components to make contact with the wafers. While this does reduce static charges, the materials currently available shed particles causing wafer contamination at the contact point. Manufacturers are seeking alternatives to this approach.
FIGS. 1-3
illustrate the above method by which wafers become charged. A cassette
11
has internal ribs
13
which loosely support a wafer
15
. Cassettes have standard dimensions designed to hold a plurality of wafers, sometimes 25 or so, for simultaneous processing through a variety of machines, such as thin film coating devices, ovens, inspection and measuring devices, and other chip manufacturing machines. The cassette
11
is open at the top and bottom but inwardly turned skirt regions
17
near the bottom provide the lowermost support for the wafers, preventing wafers from dropping through the cassette. Full details of a wafer transfer machine may be seen in U.S. Pat. No. 5,193,969 to Rush et al., assigned to Fortrend Engineering Corporation.
The cassette
11
is placed in a wafer transfer machine
21
so that the wafers can be lifted and supported from above while a second cassette replaces the first one. The machine
21
has a support plate
23
on which cassette
11
may rest, guided by side rails
25
. A pair of wafer rack members
27
project through the support plate
23
. The rack members
27
are spaced apart, insulative, polymer members which have a comb shape with upwardly projecting teeth, allowing wafers to be positioned between teeth. Motor controlled metal shafts
28
support the rack members
27
with a wafer lifting operation initiated by an operator pushing a button
30
. The objective is to raise the wafer to a gantry
29
where arms
31
will hold the wafer
15
while the lifters are retracted to a position below support plate
23
and another cassette is placed in replacement of cassette
11
.
As a wafer is raised to the position shown in
FIG. 3
, capacitance of the wafer relative to ground changes substantially compared to the capacitance in the wafer configuration of FIG.
1
. Charge induced onto the wafer in its lower position is now isolated on the wafer. The voltage associated with the isolated charge increases as capacitance of the wafer relative to ground decreases. Voltages of several kilovolts are common. With a new cassette in place, the lifters are raised until the rack members
27
contact the wafer, arms
31
are released, and the wafers are lowered into the new cassette. However, the high voltages now on the wafers have the potential of damaging wafers upon entry into a process environment.
An object of the invention was to provide an apparatus for electrostatic discharge of wafers in wafer transfer operations.
SUMMARY OF THE INVENTION
The above object has been achieved in a wafer support for particular use with a wafer transfer machine having a structure for discharge of static electricity from wafers. In particular, a comb structure with parallel teeth, spaced apart by a dimension allowing semiconductor wafers to fit between teeth, as in a rack, has an electrically grounded bus bar across the shank of the comb a slight distance, less than one millimeter, below a wafer supported by teeth of the comb. The slight distance is selected to be sufficiently close for corona discharging or electrostatic arcing to occur between the wafer and the bus bar. The bus bar is a strip fastened to the shank of the comb by cross bar members. Both the bus bar and the cross bar members are made of a high resistivity polymer having a low coefficient of thermal expansion and good machinability. High resistivity means resistivity of at least one thousand megohms per square, yet allowing full electrostatic discharge within a few seconds or less. The cross bar members are fastened to aluminum lifter shafts which act as electrical ground for the wafer support and provide mechanical lift forces for the wafer support and wafers held therein.
As wafers are raised by the lifters and a pair of comb structures, electrostatic charge accumulates on the wafers as described above. A high accumulation of charge gives rise to electrical arcing between the wafer and the bus bar on a supporting comb structure thereby discharging the wafer.


REFERENCES:
patent: 4677521 (1987-06-01), Frazier
patent: 4791524 (1988-12-01), Teigen et al.
patent: 4985719 (1991-01-01), Tsurukawa et al.
patent: 5029041 (1991-07-01), Robinson et al.
patent: 5193969 (1993-03-01), Rush et al.
patent: 5708552 (1998-01-01), Han et al.
patent: 5775752 (1998-07-01), Niemirowski et al.
patent: 5788082 (1998-08-01), Nyseth
patent: 5880924 (1999-03-01), Kumar et al.

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