Wafer center alignment device and method of wafer alignment

Details Wafer center alignment device and method of wafer alignment Wafer center alignment device and method of wafer alignment

1999-07-30

2001-05-29

Raevis, Robert (Department: 2856)

Measuring and testing

Instrument proving or calibrating

Displacement, motion, distance, or position

Reexamination Certificate

active

06237393

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to semiconductor manufacture and, more specifically, to alignment of wafers within lithography tools.
BACKGROUND OF THE INVENTION
In the manufacture of semiconductors, circuitry patterns are commonly transferred by lithography to multiple chips on a single semiconductor wafer. The lithography process generally comprises coating the wafer with a photoresist, exposing the photoresist in a pattern corresponding to the circuitry pattern desired, and developing the wafer to remove the photoresist in the exposed areas. Further processing, such as etching steps, may then follow using the patterned photoresist as a mask.
Because of the submicron-level resolution of the circuitry patterns and the constant drive to produce quality products with zero defects, the importance of aligning semiconductor wafers properly on the lithography tools is increasing. In an exemplary lithography configuration
200
, as shown schematically in
FIG. 8
, each semiconductor wafer
202
is handled by a track system
204
that performs the step of coating the wafer
202
with photoresist and then delivers the coated wafer
202
to a lithography tool
206
at a pickup-dismount location
210
. Track system
204
typically uses a robot
212
to transfer wafer
202
. After wafer
202
is delivered by robot
212
of track system
204
, a robotic component of lithography tool
206
, such as a stepper robot
214
, typically picks up the wafer
202
and takes it to a pre-aligner
216
, where the center of the wafer
202
is determined. From the pre-aligner
216
, the wafer
202
travels to the exposure chuck (not shown), where the wafer
202
is precisely positioned for alignment and exposure.
The transfer of wafer
202
from one robot to the other at pickup-dismount location
210
is critical, because the wafer
202
must be perfectly aligned at each step of the process. If the robot
212
of the track system
204
somehow becomes misaligned with the stepper robot
214
of the lithography tool
206
, then the placement of wafer
202
within the capture range of pre-aligner
216
(a range, for example a ±4 mm ×±4 mm window, within which the wafer center must be located for the wafer
202
to be further processed correctly) may be out-of-tolerance, potentially affecting final product quality. Therefore, the alignment of the robots
212
,
214
with one other at pickup-dismount location
210
must be periodically re-calibrated.
Wafer-handling robots are generally of two types: edge-handling robots that handle the wafer from the edges, and center-handling robots that handle the wafer from the center. Stepper robots are almost exclusively center-handling robots, whereas track robots may generally be edge-handling or center-handling robots. Edge-handling robots are typically aligned to a wafer center position, whereas center-handling robots are typically aligned to a wafer edge position.
Referring now to
FIG. 1
, there is shown a tilt-plate assembly
10
. Tilt-plate assembly
10
is mounted to the lithography tool (not shown in
FIG. 1
) by placement of alignment pegs
20
in corresponding indents (not shown) in the tool. Tilt-plate assembly
10
has an upper side edge
66
and a lower side edge
68
. Tilt-plate assembly
10
is typically held in place by a tensioned cross-piece
22
inserted through a hole
24
and positioned crosswise within an indent
26
. The track system robot (not shown) typically places the wafer (not shown) on tilt-plate assembly
10
with its underside touching only the tooling balls
12
on the tooling arms
14
and the tooling post
16
that extend from the tilt-plate assembly
10
.
The lithography tool robot (not show if then picks up the wafer from tilt-plate assembly
10
for further processing. A known method of calibrating the alignment of the respective robots with respect to wafer placement on the tilt-plate assembly
10
is to draw an arc on each robotic arm in the position where the edge of the wafer should be located when handled by that robot. Misalignment of the wafer edge with respect to the arc drawn on the robot arm indicates misalignment of the robots. To recalibrate, the robot positions are then adjusted by trial and error until the arcs and edges are aligned correctly. This process is time-consuming and not readily repeatable because there is no fixed reference point for both robots.
Thus, there is a need in the industry for a wafer alignment jig and a center alignment device for use with the wafer alignment jig that allow fast, repeatable calibration of the alignment of wafer-handling systems with one another. In particular, there is a need for a jig that may be used to align edge-handling wafer-handling systems and for aligning both a center-handling wafer-handling system and an edge-handling wafer-handling system to one another.
SUMMARY OF THE INVENTION
To meet these and other needs, and in view of its purposes, the present invention provides a calibration wafer for alignment of a wafer-handling system adapted to process a plurality of product wafers. The calibration wafer has radial and thickness dimensions and tolerances equivalent to the product wafers and further comprises a first center marker adapted for alignment with a second center marker external to the calibration wafer. The calibration wafer may comprise at least a portion that is transparent. The first center marker may be a visual alignment device, such as an alignment pin that may be mounted within a bushing adapted to enable movement of the alignment pin relative to the bushing along a longitudinal axis of the alignment pin.
The present invention further comprises a wafer center alignment device for precisely aligning a wafer-handling system in a calibration location with respect to a wafer-processing tool. The calibration location may be, for example, a wafer dismount location of a first wafer-handling system that is a wafer pickup location of a second wafer-handling system. The wafer center alignment device comprises an alignment jig adapted to be repeatably mounted on the tool and having a second center marker, and a calibration wafer having a first center marker. The calibration wafer is adapted to be positioned above the alignment jig with the first center marker in alignment with the second center marker. The jig may further comprise an edge-to-center locator having the second center marker on the edge-to-center locator and adapted to be mounted on the alignment fixture. The second center marker, when located on the edge-to-center locator, marks the precise center of the calibration location when the edge-to-center locator peripheral edge is positioned in contact with one or more edge stops on the alignment fixture.
The first and second center markers may be visual alignment devices, such as an alignment pin and cross hairs, mechanical alignment devices, or a transmitter and receiver. If the markers are a transmitter and receiver, the receiver may be connected to at least one of the wafer-handling systems and adapted to transmit a calibration signal to the wafer-handling system.
The present invention also comprises a method of aligning an interactive location of a wafer-handling system on a wafer-processing tool. The method comprises first mounting a pre-calibrated jig to the tool at the interactive location, the jig comprising a second center marker. Next, a calibration wafer is placed at the calibration location by the wafer-handling system, the calibration wafer having a first center marker. The alignment of the first center marker with the second center marker is evaluated and the wafer-handling system is adjusted accordingly. The wafer placement, evaluation, and adjustment steps are repeated as necessary until the wafer is placed in a position where the second center marker and the first center marker are aligned within a predetermined tolerance. The jig may further comprise an alignment fixture having one or more edge stops and an edge-to-center locator having a peripheral edge, in which case the method comprises posi

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