U.S. patent application number 11/266844 was filed with the patent office on 2006-03-16 for method and system for calibrating a laser processing system and laser marking system utilizing same.
This patent application is currently assigned to GSI Lumonics Corporation. Invention is credited to Steven P. Cahill, Jonathan S. Ehrmann, You C. Li, Kurt Pelsue, Rainer Schramm.
Application Number | 20060054608 11/266844 |
Document ID | / |
Family ID | 29550148 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060054608 |
Kind Code |
A1 |
Cahill; Steven P. ; et
al. |
March 16, 2006 |
Method and system for calibrating a laser processing system and
laser marking system utilizing same
Abstract
A method of calibrating a laser marking system includes
calibrating a laser marking system in three dimensions. The step of
calibrating includes storing data corresponding to a plurality of
heights. A position measurement of a workpiece is obtained to be
marked. Stored calibration data is associated with the position
measurement. A method and system for calibrating a laser processing
or marking system is provided. The method includes: calibrating a
laser marker over a marking field; obtaining a position measurement
of a workpiece to be marked; associating stored calibration data
with the position measurement; relatively positioning a marking
beam and the workpiece based on at least the associated calibration
data; and calibrating a laser marking system in at least three
degrees of freedom. The step of calibrating includes storing data
corresponding to a plurality of positions and controllably and
relatively positioning a marking beam based on the stored data
corresponding to the plurality of positions.
Inventors: |
Cahill; Steven P.; (Newton,
MA) ; Ehrmann; Jonathan S.; (Sudbury, MA) ;
Li; You C.; (Reading, MA) ; Schramm; Rainer;
(Everett, MA) ; Pelsue; Kurt; (Wayland,
MA) |
Correspondence
Address: |
BROOKS KUSHMAN P.C.
1000 TOWN CENTER
TWENTY-SECOND FLOOR
SOUTHFIELD
MI
48075
US
|
Assignee: |
GSI Lumonics Corporation
Billerica
MA
|
Family ID: |
29550148 |
Appl. No.: |
11/266844 |
Filed: |
November 4, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10438533 |
May 15, 2003 |
|
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11266844 |
Nov 4, 2005 |
|
|
|
60381602 |
May 17, 2002 |
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Current U.S.
Class: |
219/121.83 ;
219/121.69; 257/E23.179 |
Current CPC
Class: |
H01L 2223/54453
20130101; B23K 26/705 20151001; G06K 1/126 20130101; B23K 26/043
20130101; H01L 21/67282 20130101; B23K 2103/50 20180801; H01L
23/544 20130101; H01L 21/681 20130101; B23K 2101/40 20180801; B23K
26/361 20151001; B23K 26/04 20130101; H01L 2924/0002 20130101; B23K
26/40 20130101; B23K 2101/007 20180801; B23K 26/0853 20130101; H01L
2924/0002 20130101; B23K 26/042 20151001; H01L 2223/5448 20130101;
H01L 2223/54473 20130101; H01L 2223/5442 20130101; H01C 17/242
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
219/121.83 ;
219/121.69 |
International
Class: |
B23K 26/02 20060101
B23K026/02 |
Claims
1. A method of calibrating a laser marking system, the method
comprising: calibrating a laser marking system in three dimensions,
the step of calibrating including storing data corresponding to a
plurality of heights; obtaining a position measurement of a
workpiece to be marked; and associating stored calibration data
with the position measurement.
2. The method of claim 1 wherein the data is stored in multiple
calibration files, the calibration files corresponding to a
plurality of pre-determined marking system parameter settings.
3. The method of claim 2 wherein the multiple calibration files
correspond to a height level and one of marker system parameter
settings is a marking field dimension.
4. The method of claim 2 wherein one of the marking system
parameter settings is a spot size.
5. The method of claim 2 wherein one of the marking system
parameter settings is a working distance.
6. The method of claim 1 wherein the marking system is a backside
wafer marking system having a fine alignment camera for obtaining
reference data from a topside of the wafer.
7. A laser-based wafer marking system for marking a wafer having a
topside containing a circuit, the circuit having circuit features,
the wafer having a backside to be marked, the system comprising: a
calibrated galvanometer marking head having a scan lens and a
marking field substantially smaller than the wafer; a calibrated
positioning stage for carrying the wafer with a range of motion
large enough to position any wafer location to be marked to within
the marking field; a calibrated alignment camera with a field of
view substantially smaller than the wafer; a frame which mounts the
stage rigidly with respect to the camera and the marking head; and
a controller having a map for coordinating locations of the marking
head, stage, and alignment camera for causing the stage and the
marking head to be positioned relative to each other such that the
wafer is accurately marked on its backside relative to the circuit
features on the front side.
8. The system of claim 7 where the alignment camera and the marking
field are located on opposite sides of the wafer.
9. The system of claim 7 where the alignment camera is offset from
the marking head.
10. The system of claim 7 where a mark inspection camera is offset
from the marking field.
11. The system of claim 10 where the controller compares a location
of a mark obtained from the inspection camera with a location of a
circuit obtained with the alignment camera.
12. The system of claim 7 with a second alignment camera is offset
from the marking field and a mark inspection camera is offset from
the marking field on the backside of the wafer.
13. The system of claim 7 wherein the scan lens is a telecentric
lens.
14. The system of claim 7 wherein the controller coordinates
positioning of first and second wafer portions to be marked based
on the map, and wherein the portions overlap the marking field.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 10/438,533, filed May 15, 2003 entitled "Method And System For
Calibrating A Laser Processing System And Laser Marking System
Utilizing Same." This application also claims the benefit of U.S.
provisional application Ser. No. 60/381,602, filed May 17, 2002
entitled "Precision Laser Marking Method And System."
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to calibration of laser-based
workpiece micromachining and similar material processing systems,
wherein the workpiece has considerable warpage or sag. The
workpiece may be a semiconductor wafer, for instance a 300 mm
wafer, with a thickness of a few hundred microns.
[0004] 2. Background Art
[0005] The following representative patent references relate to
various aspects of laser marking of wafers and electronic
assemblies, illumination, and inspection/reading marks: U.S. Pat.
Nos. 4,522,656; 4,945,204; 5,329,090; 6,309,943; 6,262,388;
5,929,997; 5,690,846; 5,894,530; 5,737,122; and Japanese Patent
Abstract 11135390.
[0006] The following representative references provide general
information on various laser marking methods and system
configurations and components: "Galvanometric and Resonant Low
Inertia Scanners", Montagu, in Laser Beam Scanning, Marcel-Dekker,
1985, pp. 214-216; "Marking Applications now Encompass Many
Materials", Hayes, in Laser Focus World, February 1997, pp.
153-160; "Commercial Fiber Lasers Take on Industrial Markets",
Laser Focus World, May 1997, pp. 143-150. Patent Publications: WO
96/16767, WO 98/53949, U.S. Pat. Nos. 5,965,042; 5,942,137;
5,932,119; 5,719,372; 5,635,976; 5,600,478; 5,521,628; 5,357,077;
4,985,780; 4,945,204; 4,922,077; 4,758,848; 4,734,558; 4,856,053;
4,323,755; 4,220,842; 4,156,124.
[0007] Published Patent Applications WO 0154854, publication date 2
Aug. 2001, entitled "Laser Scanning Method and System for Marking
Articles such as Printed Circuit Boards, Integrated Circuits, and
the Like" and WO0161275, published on 23 Aug. 2001, entitled
"Method and System for Automatically Generating Reference Height
Data for use in a Three-Dimensional Inspection System" are both
assigned to the assignee of the present invention. Both
applications are hereby incorporated by reference in their
entirety.
[0008] U.S. Pat. No. 6,501,061 discloses a method of determining
scanner coordinates to accurately position a focused laser beam.
The focused laser beam is scanned over a region of interest (e.g.
an aperture) on a work-surface by a laser scanner. The position of
the focused laser beam is detected by a photodetector either at
predetermined intervals of time or space or as the focused laser
beam appears through an aperture in the work surface. The detected
position of the focused laser beam is used to generate scanner
position versus beam position data based on the position of the
laser scanner at the time the focused laser beam is detected. The
scanner position versus beam position data can be used to determine
the center of the aperture or the scanner position coordinates that
correspond with a desired position of the focused laser beam.
[0009] There is a need in certain workpiece processing systems for
calibration of multiple subsystems in three dimensions while
facilitating on-line or off-line adjustment of laser processing
parameters. Tolerance stackups within the system and workpiece may
lead to poor mark quality or mark positioning errors. For instance,
certain types of semiconductor wafers are being produced with an
increasing number of die and finer feature dimensions, while
decreasing thickness of wafers or similar workpieces have
increasing surface variations due to sag and warpage.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide an improved
method and system for calibrating a laser processing system and
laser marking system utilizing same.
[0011] In carrying out the above object and other objects of the
present invention, a method of calibrating a laser marking system
is provided. The method includes calibrating a laser marking system
in three dimensions. The step of calibrating includes storing data
corresponding to a plurality of heights. The method further
includes obtaining a position measurement of a workpiece to be
marked, and associating stored calibration data with the position
measurement.
[0012] The data may be stored in multiple calibration files. The
calibration files may correspond to a plurality of predetermined
marking system parameter settings.
[0013] The multiple calibration files may correspond to a height
level and one of marker system parameter settings may be a marking
field dimension.
[0014] One of the marking system parameter settings may be a spot
size, or may be a working distance.
[0015] The marking system may have a backside wafer marking system
having a fine alignment camera for obtaining reference data from a
topside of the wafer.
[0016] Further in carrying out the above object and other objects
of the present invention, a system is provided for laser marking of
semiconductor wafers having a pattern on a first side of the
wafers, and a second side of the wafers to be marked at
predetermined locations relative to the pattern and within a
marking field substantially smaller than the wafers. The system
includes means for calibrating a marker means of the system, and
means for controllably positioning a marking beam relative to the
wafers based on the calibration.
[0017] The system may further include an X-Y translator for
relatively positioning the wafers and the marker means for
calibrating, and means for calibrating the translator to the marker
means.
[0018] Still further in carrying out the above object and other
objects of the present invention, a laser-based wafer marking
system is provided for marking a wafer having a topside containing
a circuit. The circuit has circuit features and the wafer has a
backside to be marked. The system includes a calibrated
galvanometer marking head having a scan lens and a marking field
substantially smaller than the wafer. The system further includes a
calibrated positioning stage for carrying the wafer with a range of
motion large enough to position any wafer location to be marked to
within the marking field. The system still further includes a
calibrated alignment camera with a field of view substantially
smaller than the wafer. A frame mounts the stage rigidly with
respect to the camera and the marking head. A controller has a map
for coordinating locations of the marking head, the stage and the
alignment camera for causing the stage and the marking head to be
positioned relative to each other such that the wafer is accurately
marked on its backside relative to the circuit features on the
front side.
[0019] The alignment camera and the marking field may be located on
opposite sides of the wafer.
[0020] The alignment camera may be offset from the marking
head.
[0021] A mark inspection camera may be offset from the marking
field.
[0022] The controller may compare a location of a mark obtained
from the inspection camera with a location of a circuit obtained
with the alignment camera.
[0023] A second alignment camera may be offset from the marking
field and a mark inspection camera may be offset from the marking
field on the backside of the wafer.
[0024] The scan lens may be a telecentric lens.
[0025] The controller may coordinates positioning of first and
second wafer portions to be marked based on the map, and the
portions may overlap the marking field.
[0026] Yet still further in carrying out the above object and other
objects of the present invention, a laser based marking system is
provided for marking semiconductor substrates and the like. The
system has a laser marker with a marking field which is
substantially smaller than the substrate, a positioning subsystem
having an X-Y stage for relatively positioning the marking field,
and an alignment vision subsystem separate from the marker for
locating a feature on a substrate used to relatively position the
substrate and marking field based on a location of the feature. The
method for calibrating the system includes measuring a plurality of
fiducials disposed on an alignment target with the alignment vision
subsystem, and calibrating the alignment vision subsystem based on
the measured fiducials. The stage is positioned relative to the
alignment target to calibrate the stage using data recording the
movement of the stage and data obtained with the alignment vision
subsystem. The calibration of the stage is performed subsequent to
the step of calibrating the alignment vision system. The
calibration method further includes positioning a test substrate to
be marked, marking the substrate at a plurality of locations within
the field to obtain marks, and measuring mark locations with a
calibrated optical measurement system to obtain measurements, and
using the measurements to calibrate the laser marker, the system
thereby being calibrated.
[0027] The predetermined locations of the fiducials disposed on the
alignment target may conform to an industry standard for
measurement.
[0028] The method may further include holding the alignment target
stationary, and the X-Y stage positions at least one of marker and
the alignment vision system.
[0029] The spacing of the fiducials may be about 2.5 mm and the
alignment target may include a pattern for vision system
alignment.
[0030] The method may further include removing the calibration
target from the system and replacing the calibration target with a
test substrate to be marked. The calibration target and the test
substrate may have a substantially identical dimension and may be
positioned within a common nest in the system.
[0031] The method may further include moving the alignment target
with the X-Y stage and holding the marker and alignment vision
subsystem stationary during the step of moving.
[0032] The calibrated optical measurement system may be the
alignment vision subsystem.
[0033] The calibrated optical measurement system may further be a
metrology system having resolution substantially greater than
spacing between the marks and greater than resolution of the
alignment vision subsystem.
[0034] The above object and other objects, features, and advantages
of the present invention are readily apparent from the following
detailed description of the best mode for carrying out the
invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1A illustrates a view of the first side of
semiconductor wafer having articles, and a field of view covering
several articles; laser marking of each article is to occur in a
corresponding field on the backside of the wafer;
[0036] FIG. 1B shows an article of FIG. 1A in an expanded view;
[0037] FIG. 1C is a broken away expanded view of four articles
within the field shown in FIG. 1A;
[0038] FIG. 1D illustrates exemplary two examples of circuitry
which may be present on various articles, for instance a ball grid
array and circuit trace patterns;
[0039] FIGS. 2A-2B shows several components of a marking system of
the present invention with FIG. 2A showing the workpiece and
exemplary optical and mechanical components, and FIG. 2B depicting
a system controller;
[0040] FIG. 2C illustrates, by way of example (not to scale), ray
diagrams associated with non-telecentric alignment and marking
systems, particularly as applied to backside wafer marking based on
topside features;
[0041] FIGS. 3A-3C are a number of views wherein FIG. 3A shows a
view of the second (bottom) side of the wafer with a marking field,
corresponding to the field of view of FIG. 1A, containing the
articles of FIG. 1C; FIG. 3B is an illustration, in a broken away
view, of marks formed within a designated region on the second
side; and FIG. 3C shows an expanded view of a marked article;
[0042] FIG. 4 shows an example of a galvanometer beam positioning
system, which may be used in an embodiment of the invention for
backside marking;
[0043] FIG. 5A is a schematic diagram showing certain subsystems of
a laser marking system for semiconductor wafers for use in a
production system;
[0044] FIG. 5B is a schematic illustrating exemplary time efficient
sequencing of operations for a wafer marking process;
[0045] FIGS. 6A-6B show two alternative beam positioners, which may
be used alone or in combination for laser marking;
[0046] FIGS. 7A-7D illustrates top, end, side, and perspective
views, respectively, of a workpiece positioning mechanism for use
in an embodiment of the present invention;
[0047] FIGS. 8A-8D are top, end, side, and perspective views,
respectively, showing the use of two positioners of FIG. 7 for
supporting and positioning a rectangular workpiece (up to and
including 2 degrees of freedom);
[0048] FIGS. 9A-9C are top, side, and perspective views,
respectively, showing the use of three positioners for supporting
and positioning a round workpiece, for instance a 300 mm wafer (up
to and including 3 degrees of freedom);
[0049] FIG. 10A is a schematic representation of an exemplary laser
and optical system for general wafer marking (e.g., topside marker
shown);
[0050] FIG. 10B illustrates schematically degradation in mark
quality (e.g.: due to cracking) with increasing laser penetration
depth when compared to a mark produced using a method and system of
the present invention;
[0051] FIGS. 11A-11D relate to two and three-dimensional
calibration of the workpiece processing system of FIGS. 2A and 2B
with various calibration targets;
[0052] FIGS. 11E-11J further illustrate various calibration target
configurations for calibrating various subsystems within a laser
marking system;
[0053] FIGS. 12A-12C illustrate several features that may be
located within a field of view on a first side of a wafer, the
feature locations being used to determine a position of a marking
beam on the opposite side, for example;
[0054] FIG. 12D illustrates coordinate systems and exemplary
circuit features used for relating coordinates of a wafer to be
marked with a stored representation of the wafer;
[0055] FIGS. 13A-13C illustrate the design of a telecentric lens
for use in a precision wafer marking system with a deviation less
than about 1 spot diameter over (1) an 80 mm wide field, and (2) a
depth range corresponding to nominal wafer sag and warpage
specifications;
[0056] FIG. 14 illustrates schematically features of a laser mark
on a semiconductor wafer;
[0057] FIG. 15 schematically illustrates a wafer positioning system
wherein the wafer is initially loaded in a horizontal position, and
moved to a vertical position for alignment, marking, and inspection
operations;
[0058] FIG. 16 shows a wafer holder capable of supporting wafers in
horizontal, vertical, and upside down configurations; and
[0059] FIGS. 17A-17C show a calibration target and representative
superimposed image obtained with separate imaging systems so as to
allow for mark inspection and position verification.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Overview
[0060] Several components of a system 100 for laser marking and
inspection of wafers, for instance 300 mm wafers, is schematically
illustrated in FIG. 5A. A robot 101 transfers a wafer from a FOUP
(Front Opening Unified Portal) delivery device to a pre-aligner 102
which is used to find the notch or flat of the wafer so as to
orient the wafer for further processing. Reader 103 may be used to
extract certain coded information which in turn may be used in
subsequent processing steps. A precision stage 104 is used, and a
fine alignment procedure included to correct the residual error of
the pre-aligner (e.g., X, Y, rotation). The wafer is marked. All
marks, or a designated subset, are then inspected. In the
arrangement of FIG. 5A the inspection system is used with a
separate inspection stage 105.
[0061] A marking sequence, following opening of a FOUP, includes:
[0062] 1. Robot moves the wafer to the pre-aligner and establishes
a notch-die positional relation. [0063] 2. The wafer ID is read by
an OCR reader. [0064] 3. Mark information is obtained from a
network. [0065] 4. The robot moves the pre-aligned wafer to a
precision X-Y stage. [0066] 5. Fine X-Y-Theta alignment of the
wafer to a least correct residual pre-aligner errors. [0067] 6. The
wafer is marked using a "mark-index field-mark-index field"
repeating sequence. [0068] 7. The wafer is inspected. [0069] 8. The
wafer is returned to the FOUP.
[0070] FIG. 5B illustrates an exemplary sequence of operations for
time efficient wafer processing in a system. Various processing
steps may occur in parallel. For example, a second wafer may be
transferred for pre-alignment while fine alignment is occurring on
a first wafer.
[0071] An exemplary 300 mm wafer may have several thousand articles
(e.g.: chip scale packages, integrated circuits). The density of
the circuitry on each article can lead to difficulty in placing
machine readable marks, such as 1-dimensional or 2-dimensional
codes, in restricted areas. For instance, the die size on a 300 mm
wafer may vary from about 25 mm to 0.5 mm or smaller, with dense,
complex circuit patterns. Further, damage to circuitry which might
be caused by a high energy marking beam is to be avoided.
WO0154854, assigned to the assignee of the present invention and
hereby incorporated by reference in its entirety, discloses a
method of high resolution marking of electronic devices. Laser mark
registration is obtained from circuit features measured with a
sensor, and in one embodiment the sensor is located disjoint from a
marking head. Examples are included in '854 for marking of PCB
multi-ups and packages such as chip scale packages and die in a
tray. Sections of the '854 disclosure, including: page 4, lines
9-16, page 6, lines 1-5 and 22-29, Page 8, lines 10-17, page 9,
line 15--page 10, line 30, page 11, lines 14-20 and the sections in
the detailed description entitled "scan head", "marking operation",
and "registration" and the associated drawings of the sections are
related to the present disclosure and provide additional support
for various aspects of precision marking methods and systems
disclosed herein.
[0072] Referring to FIG. 1A, one embodiment of the present
invention provides a precision laser based method of marking a
semiconductor wafer 3, and the method may be adapted to marking of
packages, substrates or similar workpieces. The wafer 3 may have
articles 2 (one shown in an expanded view in FIG. 1B) which may
include die, chip scale packages, circuit patterns and the like.
The articles may be substantially identical, but such a restriction
is not necessary. In a typical semiconductor manufacturing
operation, subsequent to marking, the articles will be the
separated by precisely cutting ("dicing") the wafer. Further
information may be found in U.S. Pat. No. 6,309,943 wherein
alignment marks 35 (see FIGS. 3A-3C) placed on the back of a wafer
are used to define a path for precision cutting. Referring to FIG.
3, the marks 36 on an article are to be formed within a designated
region 30 relative to an article position. In this example the
backside 33 of the wafer 3 is marked near a corner of the article.
Circuits 34 correspond to a backside view of circuits 4.
[0073] In a preferred embodiment for marking large (e.g.: 300 mm)
wafers a calibration process will be used to relate an alignment
vision system coordinate (e.g., a "first side" position, for
instance at the sensor center position, and at best focus) and beam
positioning sub-system coordinate (e.g.: laser beam waist position
at the center of a marking field). Preferably, the calibration will
provide three-dimensional correction. The increasing demand for
precision placement of marks in localized areas over a large field
lead to increasing beam positioning accuracy and decreasing spot
size requirements for obtaining finer line widths or character
sizes. Over a large workpiece "sag" and warpage may be significant
relative to the depth of focus, which introduces conflicting design
parameters. Preferably, a laser spot size can be adjusted during
system operation while maintaining spot placement accuracy. With
reference to the arrangement of FIG. 2A, one embodiment includes
calibrating a first sensor sub-system 14 (e.g., a "alignment vision
system") and a beam positioner sub-system 19 (e.g., "marking
head"). The calibration is used to relate a first side position and
a marking beam position, the sub-systems each having a field of
view which is a portion of a workpiece 11 to be marked. The
workpiece may be a semiconductor wafer 3. Further details regarding
various steps of a calibration process providing both 2-dimensional
and 3-dimensional capability are provided in SECTION 1 which
follows entitled "2D/3D Calibration." Further details on various
calibration procedures for workpiece processing can be found in
exemplary references (1) U.S. Pat. No. 5,400,132 entitled,
"Rectification of a Laser Pointing Device," (2) U.S. Pat. No.
4,918,284 entitled "Calibrating Laser Trimming Apparatus," and (3)
WO0064622, "Laser Calibration Apparatus and Method."
[0074] FIG. 2C illustrates, by way of example, the multiplication
of beam position error with depth in a non-telecentric system when
marking a warped wafer 143 on the backside using frontside data
(though not so restricted). The wafer has a thickness 146, which is
typically at least a few hundred microns. Topside alignment camera
142 is shown, for the purpose of illustration, to be aligned with
marker head 147 along optical centerline 149. Planes 148,144
represent reference planes corresponding to working distances from
the marker and camera respectively. In absence of depth variations,
these planes intersect camera viewing rays and marking beams at
wafer surface positions. Reference data along ray 140 is obtained
from a reflection at the wafer surface at the point of intersection
of the wafer. The data will be, without correction, represented as
a coordinate corresponding to the intersection with plane 144,
which is to be related to a marking coordinate. A lateral position
error 1400 results. Assume for the purpose of illustration a mark
is to be placed on the back of the wafer at a position
corresponding to reference data taken along ray 140 at the wafer
intersection. A marking beam, without correction, will be directed
to a point in the plane 148 corresponding to the reference data
(and position error). However, this may result in a mark outside of
a designated region, as shown by the direction of central ray of
marking beam 141 at the actual intersection point with the
wafer.
[0075] The three-dimensional calibration process of SECTION 1 of
the Appendix, with suitable height measurements of the wafer, may
be used to determine a correction to be applied to the beam
positioner.
[0076] In a preferred telecentric system, the error is reduced to
about 1 spot or finer with a lens (see SECTION 5 which follows
entitled "Precision Telecentric Lens") of low to moderate cost.
Preferably, the telecentric design compensates for the worst case
wafer warpage and additional system "stackup" errors. With the
preferred arrangement a field size supporting relatively high
marking speeds is maintained. In the telecentric case the
calibration process may be streamlined, but multiple calibration
files used to at least control and maintain the laser spot size
over the working volume are preferred. This provides for consistent
marks and for mark contrast control.
[0077] Three-dimensional tolerances are to be considered for the
alignment and marking sub-systems in view of the workpiece
variations relative to the depth of focus of the optical systems.
Increasing the alignment system magnification to improve feature
location accuracy decreases the depth of focus. Various focusing
methods are useful to position the entire sub-system 14 and/or lens
system 15 (shown as a telecentric lens but not so restricted)
relative to the workpiece along the Z-axis. For example, the Z-axis
position corresponding to the maximum edge contrast at a die
location is a possible measure. A measurement of the maximum
intensity of a "point" or small target (one the order of a pixel)
may provide more sensitivity to depth changes.
[0078] Wafer "sag" is somewhat predictable from a specification of
wafer thickness. Predictions based on models (fixed edge and simple
support) with wafer thickness ranges of about 300 .mu.m to 775
.mu.m indicated about 60 .mu.m of deviation for the latter case.
For thinner wafers the deviation increases, and the overall
deviations may be further increased by warpage and other stackups.
Surface deviations may be estimated and used for certain
correction. A telecentric system, for instance as described in
SECTION 5, is predicted to yield less than .mu.m of spot placement
error over a 4''-marking field. Various sub-systems, including the
scan head, alignment vision system, and perhaps inspection system
may include at least an option for height sensing. Similarly, a
separate sub-system could be added specifically for height
measurements at a plurality of locations on the wafer surface.
Preferably, any degradation in the cycle time of the machine will
be negligible.
[0079] In one arrangement the alignment vision system 14 will be
relatively positioned at sample points which may include but are
not limited to the regions used for feature detection. As mentioned
earlier, the focus sensing may be achieved by sampling the image
contrast at locations along the z-axis using the alignment vision
system. The z-axis locations are recorded. Alternatively, a
triangulation or focus sensor, which may be a commercially
available module, may be used for measuring surface points which
are used with the alignment and calibration algorithms (and the
known wafer thickness) to map the surface. Similarly, a direct
measurement of the second side may be obtained with a sensor
included with the vision inspection module 20. In an alternative
arrangement a "full field" system, for instance a commercially
available Moire Camera, may be used. In any case, the data will
preferably be used to position the marking beam waist at the
surface. In accordance with the preferred calibration method of
SECTION 1, the desired spot size will be maintained at the marking
locations. In one arrangement the marking beam waist may be
positioned in discrete steps, for instance at 9 locations within an
80 mm field for center-edge compensation. Non-contact optical
sensing is preferred, but capacitance or touch probes may be
acceptable.
[0080] If the deviations correspond to a simple second order curve
and are symmetric, then the wafer surface may be sampled along a
diagonal direction using at least three locations (edge region,
center, edge region). If warpage is represented with a higher order
curve (e.g.: "potato chip") additional data will be acquired, for
instance at least nine locations. If the data is acquired with the
first side alignment system, the second side location may be
approximated using the thickness of the wafer, which may be
measured or specified by the operator.
[0081] Similarly, both for calibration and marking, a marking beam
focus function may be sampled at a number of locations in the
marking field (at reduced power). The system may include a
detection system suitable for measuring "featureless" surfaces, for
example a bicell or quad-cell arrangement. Alternatively, a
projected grid may be used similar to the options provided in
commercially available Metrology equipment manufactured by Optical
Gaging Products (Rochester, N.Y.). The focusing tool will
preferably be used for both alignment and system setup operations
in addition to measuring the working distance during wafer
marking.
[0082] In the system of FIGS. 2A and 2B, both the beam positioning
subsystem and the alignment system preferably include telecentric
optical systems 351 and 15, respectively, which reduce or eliminate
variation in the position of an angular scanned marking beam
position with depth. SECTION 5 shows a telecentric lens system
which provides spot placement accuracy better than one spot
diameter over a field size of about 80 mm, and over a depth range
corresponding to worst case expected sag/warpage. The 80 mm field
allows for significantly higher marking speeds compared to smaller
non-telecentric fields. Furthermore, the 30 .mu.m spot size is
finer than most wafer mark systems, a desirable feature for
controlling mark contrast and resolution.
[0083] However, other alternatives may be used with appropriate
compensation for positioning with depth. For instance, in one
embodiment a telecentric lens 15 may be used, but an arrangement
similar to 47 of FIG. 6 may be used for marking (as discussed
below).
[0084] With reference to FIG. 2A, the preferred alignment
sub-system will have a high resolution camera 13, for example a
1280.times.1000 CCD imaging array with image processing hardware
and software for extracting and processing smaller regions using a
"software zoom" feature. Alternatively, a calibrated "zoom" optical
system may be used. Illumination system 21 may include special
illumination design, for instance a combination of dark and bright
field illuminators, to enhance the contrast of features used for
alignment. In one embodiment an LED array provides low angle
illumination, with a manually adjustable angle. In the
configuration using the high resolution camera the exposure is
fixed which simplifies the design, eliminating the dependence of
the image "brightness" with magnification.
[0085] In one embodiment, the marking sub-system 19 includes the
system shown in FIG. 4 with X-Y galvanometers providing deflection
system 40, 41, 42, 43 and possibly a beam expander assembly 49.
FIG. 6, incorporated from the earlier cited reference to Montagu,
pp. 227-228 shows alternative pre-objective 46 (e.g.: telecentric)
and a post-objective 47 scanning arrangements, the latter
incorporating an additional dynamic focus translator 48. In a
preferred telecentric system of the present invention components
may be included for dynamic focus 48 and/or spot size adjustment
with a computer controlled version of expander 49 of FIG. 4.
[0086] The fine alignment system provides correction for residual
X-Y-angle errors associated with the transfer and pre-aligner. In
one embodiment wherein only small variations occur or are
specified, the alignment system may correct X, Y, and theta (e.g.:
angle) variations with measurements taken at three locations (e.g.
fiducials). However, with emerging tight tolerances an increasing
density of circuit/wafer, increased accuracy is preferred. The fine
alignment system of 14 provides added capability of recognizing
and/or measuring features associated with an article 2 of the wafer
(e.g.: machine vision/pattern recognition capabilities). A feature
location will be determined. An algorithm is used to obtain
reference data and to locate a feature associated with at least one
article 2 on a first side of the workpiece 3 using at least one
signal from the first sensor 13. For example, article 2 of FIG. 1A,
shown in an expanded view of FIG. 1B, may have a circuit pattern
with detectable conductor traces 7 or pads 5 which may be
replicated 4 in at least a portion of the wafer (but not
necessarily over the entire wafer). In a preferred system, a
pattern recognition algorithm will, based on "training" on a
reference wafer, for instance, automatically learn at least a
portion of the workpiece structure and determine the relative
location of the pads, traces, or similar features. For instance,
the rectangular outline of a die (article) 6 or corner locations
may be used as one feature to locate the die edge and/or estimate
the center. The location may be related to a location of at least
one other die in 4 located within the marking field 1 of FIG. 3A,
or possibly outside the field if tolerances permit. For example, a
minimum of 3 non-collinear locations are determined over the
workpiece and used to calculate an offset and rotation correction
for the entire workpiece. Another pattern may be defined by the
location of an array of solder balls or pads 8 as an
alternative/equivalent. Yet another pattern may include sections of
internal circuitry of the article having even greater density than
illustrated in FIGS. 1A-1D. The algorithm may include matching
features of the workpiece using a machine vision sub-system, for
instance a grey scale or binary correlation algorithm. Various
"modules" and algorithms for pattern recognition and matching are
commercially available (e.g.: Cognex Inc.) which may be adapted for
use with the present invention. The workpiece may have identical
and repetitive patterns.
[0087] In a preferred arrangement the matching is automatically
performed over all the articles, and without human intervention. It
should be noted that many combinations of patterns may be present
on a wafer with special marking requirements (e.g.: "binning") and
the preferred algorithm will have substantial flexibility. The
"training" may further include a semi-automatic, operator guided
teaching phase so as to efficiently program the machine for
recognition and matching of complex patterns.
[0088] In WO 0161275, incorporated by reference and assigned to the
assignee of the present invention, various detection and
recognition algorithms are disclosed for automatic learning of
circuit features using grey-scale and/or height information, and
subsequent use of the stored information for inspection. For
instance, the following sections of the '275 disclosure: page 7,
lines 4-26; page 8, lines 1-5 and lines 17-25; page 9, lines 5-10;
page 10, lines 24-25; page 11, lines 1-18; page 15, lines 29-30;
page 16, lines 1-10; page 17, lines 19-28 and the associated
drawings teach the application of various pattern recognition and
learning algorithms. Further details of various steps for detection
and matching features for obtaining reference locations for
precision marking of wafers and similar articles are disclosed in
SECTION 2 which follows entitled "Feature Detection and Fine
Alignment."
[0089] In one embodiment of a 300 mm marking system, an 80 mm
marking field is used for high speed, and an alignment vision field
of approximately 16 mm is used to for feature detection. With a
1024.times.1024 array a 16 .mu.m pixel size will be provided, which
is somewhat finer than the spot size of the marking beam. For
example, in an embodiment of a backside wafer marking system a spot
size of less than 40 .mu.m is preferred, with a most preferred
range of about 25-35 .mu.m. The marking field 1 dimension (depicted
in FIG. 3 and corresponding to the region 4 of FIG. 1-A but on the
backside 33 of wafer 3) may be a relatively small fraction of the
workpiece 3 dimension (e.g.: a 300 mm maximum wafer size in a
system configured so as mark wafers of varying specified
dimensions). For example, in one embodiment for marking 300 mm
wafers nine or more marking fields having dimensions in the range
of about 75-100 mm are used to provide marking precision and high
speed operation. In a case where a workpiece is severely warped,
the marking field may be reduced by controlling the amplitude of
the scan angle, based on surface measurements or a specification.
Precision marking includes relatively positioning the beam
positioner sub-system 19 (or a component of the sub-system) and the
workpiece 11 so as to position a laser beam at a marking location
30 on a second side of the workpiece 33 as shown in FIG. 3, the
positioning based on the feature location on the first side. The
feature location may define the location of the article (e.g.: edge
or center) or otherwise be related to designated region(s) 30 for
marking located on the second side. Various methods and sub-systems
may be used for the positioning as described in more detail
below.
[0090] As shown in FIGS. 3A-3C, a predetermined code or other
machine-readable indicia 36 is marked on the workpiece, typically
with a scanned laser marking output beam (vector or dot matrix, for
instance) within the field defined by 24 of FIG. 2A, preferably
using telecentric lens 351. A machine readable mark is formed in
the designated region. Also, laser induced damage to an article 2
is avoided by marking the second side
[0091] The steps of obtaining reference data, relatively
positioning, and marking are repeated so as to locate a feature
associated with at least one article on the first side, and to
position a marking beam within all the designated regions on the
second side based on the feature location(s).
[0092] The beam positioning sub-system preferably includes a 2D
galvanometer scanner 40, 41, 42, 43 as shown in FIG. 4 (but
preferably adapted for irradiating the workpiece with a telecentric
beam as shown in FIG. 2A and approximately as in arrangement 46 in
FIG. 6). Alternatively, the sub-system may include a translation
stage or rotary stage with beam delivery optics. The laser and
optical system may be integral or remotely coupled, for instance
with a fiber delivery system. The field of view of the beam
positioner may range from a few laser spot diameters to a
relatively wide angular field, but for precision marking in
accordance with the present invention the field will be a portion
of the largest workpiece to be marked in the system. For example,
wafers of 100, 200, and 300 mm may be marked and the marking field
1 dimension (e.g.: first side view in FIG. 1A, second side view in
FIG. 3A) may be about 100 mm. In certain cases a pattern may be
marked on workpiece (say with a lower laser power requirement) with
parallel beams as illustrated in publication WO961676, and/or U.S.
Pat. No. 5,521,628. Various combinations of serial and parallel
operation may be used, for instance with multiple marking heads as
taught in U.S. Pat. No. 6,262,388. The 2D/3D calibration process of
the present invention may be adapted to these marking
configurations to maintain accuracy.
[0093] Relatively positioning may further include: (i) providing a
beam positioner which may include a 2D galvanometer deflector; (ii)
adjusting a mirror 42, 43 position (See FIG. 4) if the marking
location is within the field; (iii) relatively translating the
workpiece 11 and beam positioning sub-system 19 so as to position
the location within the marking field 1 whenever the location is
outside the marking field. The features related to article 2 (also
depicted by the dashed lines of FIG. 2A) are used as discussed
above to determine a position of the marking beam, and the position
will preferably be a three dimensional coordinate. Further, the
specified or measured thickness of the wafer may be a parameter
used to determine the focal position of the beam relative to a
front side position.
[0094] In a preferred system for wafer marking at least one
workpiece positioner is used in addition to stage 18 (also depicted
as 104 in FIG. 5A) for fine positioning. The positioning sub-system
is configured so as to support and position workpieces 11 of
varying specified dimensions, while allowing radiation beams
(marking beam(s) over field 24 and illumination/viewing beams in
fine alignment camera field 25 from light source 21) shown in FIG.
2A to directly irradiate the first and second sides of the
workpiece. In one embodiment, a wafer chuck 17 (see SECTION 3 which
follows entitled "Workpiece Chuck/Positioner") is provided with a
Z-axis (direction 26) drive with an option of smaller wafer inserts
to support the wafer or other workpiece. The system is preferably
automated with an arrangement of end effector(s) transferring the
workpiece to the chuck 17 which automatically clamps, grips, or
otherwise supports (shown in a single schematic view in FIG. 2A)
the workpiece. Surface damage and significant distortion are to be
avoided.
[0095] In view of the aforementioned emerging three-dimensional
variations and tolerance requirements, it is preferred that the
marking beam focus position shown as 422 in FIG. 4 (e.g.: beam
waist) and attitude (roll, pitch relative to the focal plane)
depicted by the arrow 22 (see FIG. 2A) be adjustable. For example,
variations in "sag" or warpage of the wafer in addition to stackup
tolerances may be compensated by providing a total adjustment range
of at least about + or -2 millimeters. Referring to FIG. 2A, the
adjustment may include relative Z-axis (depth) positioning of the
laser beam positioning sub-system 19 and workpiece along a
direction substantially perpendicular to the workpiece so that the
beam waist of the laser substantially coincides with the workpiece.
The adjustment may be dynamic and done for each wafer. The
adjustment may include tilting 22 (pitch, roll) of the laser beam
positioner and/or workpiece to so that a focal plane of the laser
beam is substantially parallel to a local planar region of
workpiece (e.g.: over a marking field). Alternatively, a planar
region may correspond to a best fit plane over the workpiece. Some
adjustments may be done with a combination of manual or
semi-automatic positioning of the beam positioner, for instance
during calibration or setup. Similarly, the end effector(s) and the
chuck 17 coupled to precision stage 18 may be controlled by a
program so position the workpiece 11 in angle (roll, pitch) and
depth. SECTION 3 of the Appendix illustrates specific details of an
embodiment for automatic precision positioning of a circular (for
instance a 300 mm wafer) or rectangular workpiece with actuators
for adjustment of the height and preferably attitude. The
arrangement is particularly adapted for height adjustment. Various
modifications, for instance spherical or point contact at the
support base 53 in FIGS. 7C and 7D, will facilitate the fine
angular positioning (roll, pitch) of the workpiece, for instance,
tilting wafers having thickness of 300 .mu.m or less.
[0096] In an alternative arrangement the wafer may be held in a
vertical position. For instance, a suitably modified and automated
version of the "Wafer Edge Fixture" produced by Chapman
instruments, and configured for a maximum wafer size 300 mm
(Chapman Instruments, Rochester, N.Y., and referenced to U.S. Pat.
No. 5,986,753) may be used. Six degrees of freedom are included for
profiling of wafers. Further description of the tilt stage, wafer
chuck, X-Y-Z stage, and controller are found in the article "Wafer
Edge Measurements--New Manual Fixture Provides More Features."
[0097] In one embodiment for wafer marking a "split gantry" stage
is an alternative with automatic positioning of the horizontal
mounted marking head along one direction (e.g.: "X", horizontal,
into the page) and wafer positioning in at least a second direction
(e.g.: "Y" vertical and along the page, and "Z" along the optical
axis, and preferably including capability for roll and pitch
adjustment).
[0098] FIG. 15 illustrates a perspective view of yet another
positioning arrangement with several components marking system also
illustrated. The wafer is translated in two dimensions (e.g.:
translation in a plane perpendicular to the page of FIG. 2A). The
wafer is oriented with an end effector to notch 702 and loaded into
holder 701. A hinge 703 is used for loading in the horizontal
position followed by transferring to a vertical position for
marking with a beam incident through scan lens 351. At least two
axes of motion 704 and 705 are provided. The construction allows
for marking the backside and for fine alignment using camera 13
wherein the location of front side features are used to position
the marking beam.
[0099] FIG. 16 shows details of one arrangement for holding wafers
at various orientations. In this arrangement wedge 800 is engaged
by a spring 801 held open by vacuum so as to allow for mounting in
a horizontal, vertical, or upside down orientation.
[0100] Various combinations of the motion (manual or automatic) of
the (1) workpiece positioner 18 and (2) beam positioning sub-system
(e.g.: "marking head") 19 and/or (3) internal components of 19
(e.g.: a dynamic focus sub-system 48 and/or beam expander 49 may be
used and coordinated with controller 27. For instance, five axes of
motion (e.g.: X, Y, Z and Roll, Pitch) may be implemented for
precision positioning in a wafer processing station 100. Further,
coarse (possibly manual or semi-automatic) positioning may be
implemented in one or more axes, for instance.
[0101] The selection of laser pulse characteristics can have a
significant effect on the speed, contrast, and overall quality of
the marks. For backside marking of Silicon wafers a pulsewidth of
about 15 ns, repetition rate of about 25 KHz, and output energy of
about 0.23-0.25 millijoules at a wavelength of 532 nm provided
favorable results. A short cavity green Vanadate laser was used.
Further, marking depth penetration of about 3 .mu.m-4.5 .mu.m
provided machine readable marks without internal damage (e.g.:
cracking) of the wafer. Marking speeds of about 150 mm/sec were
achieved, and it is expected that about 350 mm will be achievable
with preferred laser parameters. The marking speed represents a
relative improvement for marking in view of the large number of
articles to be marked at high resolution. An exemplary range of
operation includes pulse width of about 10-15 ns, repetition rate
of about 15-30 KHz, with focused spot size of about 30-35 .mu.m for
marks on Silicon wafers. Another range may include a pulsewidth of
up to about 50 ns, and a minimum repetition rate of about 10 KHz.
Micro-cracking is also prevented by limiting penetration of the
beam to a depth of less than about 10 .mu.m. It is expected that a
wavelength of 1.064 .mu.m will be suitable for marking metal
workpieces, with frequency doubled operation for Silicon wafer
marking. Further details on a preferred laser and associated
characteristics are disclosed in SECTION 4 which follows entitled
"Laser Parameters and Mark Quality."
[0102] Referring to FIG. 2A, a vision inspection system 20, viewing
the second side, will generally include an illuminator, camera or
other imaging device, and inspection software. In a preferred
system the inspection field is calibrated to the fine alignment
vision field. For instance, the centerlines may be aligned 29 as
shown in FIG. 2A, with a large overlap between the fields. This
provides for overlaying the marks on the die for mark manual or
automatic visual verification. SECTION 6 which follows and is
entitled "Backside Mark Inspection With Frontside Die Registration"
describes details of an embodiment for inspecting marked wafers.
All the marks (100% inspection) may be inspected, or a
user-specified subset. For example, a few locations on the wafer
may be marked and the results analyzed. If the results meet
specifications all the remaining designated regions of the wafer
may be marked. The vision system may be mounted on a separate stage
wherein a first wafer is inspected while a second is marked (See
FIG. 5A). FIG. 2A illustrates an alternative arrangement wherein a
single stage 18 is used to position the workpiece for both
inspection and marking.
[0103] The inspection system will preferably provide feedback
regarding mark quality as rapidly as possible to maximize yield.
For instance, a wafer may have 30,000 chip scale packages as
articles. A marking field may have at least a thousand die. A
separate inspection system with "standard" lighting for viewing
marks may be an advantage to establish correlation between various
stages of the wafer and device assembly steps wherein the marks may
also be viewed. In an embodiment where the inspection system
optical axis is separated the inspection may occur in a sequence
where a first field is marked and then inspected. The inspection of
the first field will occur while a second adjacent field is being
marked when a large number of articles are to be inspected.
[0104] In an embodiment using a pair of galvanometer mirrors, data
representing at least a sample of die (or other article) over the
field may be acquired with a "through the lens" vision system
(e.g.: a second simpler vision system for the case of wafer mark
inspection). The data processing operation may overlap with
positioning (indexing) to an adjacent field. It should be noted
that the coaxial vision system might not require a vision system
with complete inspection capability. For instance, the intensity or
radiation pattern of the reflected scanned beam may be analyzed for
early detection of gross mark defects or other process problems.
For instance, a single photodetector may be used to analyze the
reflected marking beam. Telecentric viewing (e.g.: received through
lens 351) reduces variations with angle, which can provide for
improved classification of signals.
[0105] Some Further Discussion of Various Alternatives:
[0106] In a preferred embodiment the workpiece 11 is translated
when indexing to marking fields. However, the relative motion of
the workpiece 11 and beam positioning sub-system 19 may include
translation of at least a portion of the beam positioner (or a
component). When marking wafers, a single X-Y stage moving the
wafer allows for positioning of the alignment system 14, marking
lens 351, inspection system 20, and possibly an optional mark
verification reader. In an embodiment wherein the wafer is
translated, alignment and beam scanning may be simplified. In an
embodiment where the positioning sub-system or portion of the
sub-system is translated fiber beam delivery from a remote laser
source to marking head 19 may be used to an advantage.
[0107] In one embodiment for wafer marking a Z-axis stage 28 may be
used. A range of at least + or -2 mm is preferred. The beam
positioner 19 and lens 351 may move, but movement of the wafer is
preferred. The Z motion may be determined by the focus of the
alignment camera system components 13, 15. The sag and warp of the
wafer is preferably compensated by movement (translation, roll,
pitch) of the wafer with the positioning system 18, 17 or by
movement of the beam positioner 19 as described above.
[0108] A total Z range of travel of about 12 mm, implemented with
one or more translators, may be used to allow a robotic end
effector to load a wafer while allowing for compensation of wafer
sag by relative movement of the wafer and marking beam focus
location.
[0109] A method for controlling contamination may be an advantage.
For example, a tilted window, placed between lens 351 and the
workpiece, with a slight amount of vibration may remove particles
from the marking lens. Air pressure may be used to clean the lens
during idle periods. A tilted window will displace the beam and
aberrations may be introduced. Certain errors (e.g.: beam
displacement) may be corrected during calibration. Alternatively,
an "air knife" may be used to produce fast moving air across the
lens.
[0110] An exemplary exclusion zone of about 2-3 mm is typically
used.
[0111] The wafer nest may have vacuum applied on the 2 mm exclusion
zone. The nest may be held with a kinematic mount.
[0112] The focal position of the alignment system lens 15 and
camera 13 may be used for determining a Z-axis location and for
fine positioning of the beam. In one embodiment the wafer is
translated. Alternatively, the camera system may be focused and the
position recorded. The position my then be related to the beam
positioner coordinates (e.g.: the lens position) and the lens and
positioner translated accordingly.
[0113] In one embodiment slight relative movement of the Z-axis
position may be used to compensate for sag and warp. For instance,
a change in the z-axis position may be effected at a plurality of
marking locations over a 100 mm marking field. For instance,
Z-translation may occur at nine locations (e.g.: to compensate from
center to edge).
[0114] The X-Y table may have a range of travel of about 12-18
inches, with linear encoders for position feedback.
[0115] An inspection module may have optical resolution of about 4
microns.
[0116] A telecentric lens may be used with the fine alignment
system.
[0117] The inspection module 20 may also be used for certain
alignment operations (e.g. locating a fiducial on backside) and may
be calibrated using a transparent alignment target to establish
correspondence with the coordinate system of the fine alignment
camera 13.
[0118] The recommended marking depth for optimum reading, while
avoiding substrate damage, may be about 3.5 microns. The laser
system may be configured for a maximum mark depth of about 10
microns.
[0119] Embodiments of the present invention may be used to mark
wafers with programmable field sizes and number of fields (e.g.:
9-16 fields of view on a wafer having a diameter in a range of
150-300 mm), focusing options (e.g.: 3 focus positions for wafers
775 microns thick with increasing density for thinner wafers), and
various marking speeds (e.g.: 150-250 mm/sec).
[0120] Various exemplary and non-limiting system parameters and
associated tolerances may include: TABLE-US-00001 PARAMETER
TOLERANCE Encoder Resolution .1 microns Z-stage Travel 10 mm
Z-stage Perpendicularity .1 mRad Z-atage Accuracy +/-5 microns Fine
Alignment Repeatability 1-2 microns Spot Size .ltoreq.60 microns
nominal, 25-40 .mu.m preferred Galvo (calibrated field) +/-30-50
micron accuracy Marking Lens Option (due to sag) telecentric, +/-3
micron, 300 micron wafer thickness, 300 mm wafer Marking Lens
Option flat field, +/-10 micron, 775 micron wafer thickness, 300 mm
wafer
[0121] Numerous alternatives may be used to practice the invention.
Variations of the positioner type, number of positioners, vision
systems, focusing hardware, laser types including q-switched and
fiber lasers, may be used. Furthermore, the choice of
serial/parallel operation of multiple markers and inspectors for
efficient production time management and yield improvement,
including cluster tools and statistical process control may be
incorporated for use with a precision marking system of the
invention. Further, it is contemplated that the pattern recognition
and marking techniques of present invention may be used alone or in
combination with other production processes, for instance the
"dicing" operation described in the aforementioned '943 patent.
SECTION 1-2D/3D Calibration
[0122] Various commercially available marking and workpiece
processing systems calibrate the laser marking field by marking a
grid on test mirror and measuring the grid on a separate coordinate
measuring or metrology machine. It is an iterative process and very
time consuming. Other laser systems use the on-line
through-scan-lens vision system to calibrate the laser-marking
field on the same side. Alternatively, a substrate or disposable
workpiece may be marked.
[0123] In accordance with the present invention, "two-dimensional
calibration" utilizes an x-y stage, a pair of stages translating
the workpiece and/or marking head, or other arrangement which
allows the on-line machine vision sub-system 14 of FIG. 11A to
calibrate the laser marking field 24 on the OPPOSITE side. The
calibration is used to mark the second side based on vision data
and features from the first side.
[0124] Calibration may be system dependent and manual, automatic,
or semi-automatic. By way of example, four steps for calibration
are shown below to illustrate aspects of overall system
calibration: [0125] 1. Calibrating camera pixels for each camera in
system. [0126] 2. Calibrating coordinates of a first camera to a
second camera. [0127] 3. Calibrating stage coordinates to camera
coordinates [0128] 4. Calibrating the scan head to the wafer
nest.
[0129] FIG. 11E schematically illustrates a typical arrangement for
respective top and bottom cameras 501 and 502. In at least one
embodiment of the present invention each camera is calibrated
separately to match the camera pixels to actual "real world"
coordinates. FIGS. 11F and 11G schematically illustrate a "tool
area" 505, which is relatively positioned within camera 501,502
fields of view. Preferably, the cameras may be mechanically
positioned within the system so the fields of view substantially
overlap, but the fields may be separated. In one exemplary
arrangement the crosshairs 506 may be about 5 mm apart. The
calibration may include measuring the coordinates of the crosshairs
and estimating a center position, scale factors, and rotation of a
coordinate system relative to the tool. Preferably, at least the
"pixel size" of the camera will be measured. Alternative
embodiments may include additional crosshairs of other suitable
targets and calibration of sub-fields within the camera field of
view.
[0130] FIG. 11H illustrates a calibration step wherein the top and
bottom cameras preferably view (simultaneously) target 511 as seen
by a first camera and the same target depicted by dashed lines 510
as seen by a second camera. The calibration target may be within
the "tool area" as shown. A correction for offset, scale, and
rotation is applied. In one embodiment an additional crosshair may
be used to specify the center of the object. This arrangement, with
precision calibration, is particularly useful for providing a
display showing a mark on the backside of a wafer relative to a die
position as seen on the front side for the purpose of mark
inspection (see SECTION 6).
[0131] Yet another calibration step may be applied to compensate
for X-Y stage tolerances. FIG. 11I illustrates three crosshairs 520
used for calibration wherein the entire nest is moved and camera
coordinates are related to stage coordinates. As such, the
tolerance stackup of the stage is compensated.
[0132] Yet another calibration may be applied to calibrate the scan
lens 351 of marking head 19 in FIG. 11A to stage coordinates. FIG.
11J shows a consumable part, for instance a black anodized disk 521
which may be marked with five crosshairs, one shown as 522.
Software is used to inspect the marked plate. The marking field may
be a fraction of the disk 521 size, and an X-Y stage provides for
relative positioning of the disk and marking beam.
[0133] These basic steps above may be sufficient and preferred in a
system wherein marking performance is substantially invariant with
depth (e.g.: large depth of focus, relatively large laser spots,
relatively small wafers having exemplary thickness of about 775
microns and minimal sag).
[0134] In one embodiment the alignment vision subsystem 14 of FIG.
11A may be calibrated first with a previously marked wafer or
alternatively with a precision grid (e.g.: each preferably
conforming to a calibration standard). For instance a 200 mm wafer
or other maximum wafer size to be marked with the system may be
used. The wafer marks may include with a grid of fiducials similar
to a crosshair 522 of FIG. 11J. In one embodiment the wafer has a
77.times.77 array of crosshairs with 2.5 mm spacing with a special
pattern at the center of the grid. The camera focus is preferably
checked (e.g: contrast measurement) over the grid and mechanical
adjustments made to the nest. Alternatively, a positioner (e.g.:
see FIGS. 9A-9C) may be adjusted in depth or attitude if used in a
system. The marked calibration wafer is also used for a next
calibration step wherein the X-Y stage 18 is calibrated. The
initial X-Y stage calibration may take several hours to complete
with calibration over the range of travel, the calibration
information being recorded by imaging a crosshair or other suitable
target on the calibration wafer. The data is then evaluated. A
third calibration step of the embodiment is a marking field
calibration wherein a 200 mm wafer (or maximum size wafer to be
marked) is marked with a pattern similar that of FIG. 11J, or other
pattern with suitable density. Preferably, the X-Y stage is
calibrated as above prior to calibration of the marker. The mark
positions are then measured with using the fine alignment camera,
or with a separate vision subsystem. For example, the marks may be
measured with a commercially available, "off-line" precision
Metrology system produced by Optical Gaging Products (OGP), for
instance a Voyager measuring machines. If marker field calibration
is to be periodically repeated as part operation of the marker in a
production environment, the alignment vision system may be used.
Preferably, the resolution and accuracy of the alignment system
will substantially exceed the minimum mark spacing.
[0135] Compensation for workpiece sag and warpage may require
maintaining the same spot size with different working distances.
Besides, there is an increasing need to change laser beam spot
sizes during operation to meet different application parameters,
such as line width, character size, mark contrast, hard-mark,
soft-mark, throughput, etc. Three-dimensional calibration provides
calibration at a plurality of marking positions along the Z-axis.
As a result, the laser marking field capability is provided for
changing the laser beam working distance and/or spot size
automatically while maintaining the laser beam position
accuracy.
[0136] There is also an increasing need to change the size of the
field of view (FOV) of the machine vision system during operation
to meet different application requirements. Three-dimensional
calibration on machine vision allows the system to change the size
of FOV automatically and maintain the vision dimension accuracy at
the same time.
[0137] Referring to FIGS. 11A-11D, in one arrangement, a
two-dimensional calibration procedure relating a position of the
first side to the laser marking field 24 on the second side
includes a calibrated machine vision sub-system 14 and calibrated
x-y stage 18 that will mark a mirror 92 (one mark shown as 95 in
FIG. 11B). A description of the calibration of stage 18 and camera
sub-system 14 is shown below (steps 1 and 2). The test mirror is
positioned at a predetermined working distance with coated surface
facing the laser source. The marking laser beam 93 is directed to
several locations on the surface so as to mark 95 an N.times.N grid
on the mirror 92. In the illustrated embodiment the x-y stage 18
moves the mirror in both x and y directions so that the alignment
vision camera 13 can "see" each node on the grid from non-coated
surface of the mirror (opposite side from laser source).
Illumination from light source 21, or other suitable illumination,
is used and depicted by illumination beam 94. The coordinates of
each node are recorded. A calibrated algorithm or look up table is
then generated relating the coordinates.
[0138] The calibration techniques described herein are not
restricted to "topside" imaging and "bottomside" marking. For
example, the process may be applied to wafer marking in a system
where a chuck holds the wafer in a vertical position, and the
marking and illumination beams are substantially horizontal.
Likewise, the workpiece may be marked from the topside based on
calibration and reference data from the bottom-side. Similarly, the
process may be adapted for calibrating separated alignment and
marking fields, both covering regions of a single side of a
workpiece.
[0139] In order to optimize the system for different application
parameters, sometimes one or more machine settings might require
adjustment during the operation. When the change in setting affects
the system accuracy, a new calibration will be required. The
three-dimensional calibration process is used to create multiple
layers of calibration files with respect to different system
settings. A three-dimensional calibrated system can switch between
different settings automatically and achieve the required
performance and accuracy by using the corresponding calibration
files. Exemplary methods to achieve three-dimensional calibration
for different settings on the system include:
[0140] 1. Laser beam spot size versus laser working distance: Use
z-stage 28, and/or a combination of relative motion of chuck 17,
and/or motion of an optical sub-system within marking head 19 to
relatively position the test mirror to different working distances
with respect to the laser source. Varying the working distance
de-focuses the laser beam and provides different spot size at the
work surface. It has been determined that a defocused spot provides
acceptable mark quality for certain workpieces, and hence is
considered. The two-dimensional calibration described above is
repeated for each working distance. As the result, a group of
calibrated algorithms or look up tables for different spot sizes
with corresponding working distances is generated.
[0141] 2. Laser beam spot size versus laser beam expander setting:
Use an expander for focus control, zoom expansion control, or the
combination. For instance, a computer controlled embodiment of the
expander 49 shown in FIG. 4 may be used to achieve different laser
beam spot sizes on a work surface at fixed working distance.
Different combinations of laser beam expansion and focus can be
used to achieve a desired spot size. Then the two-dimensional
calibration described above is repeated for each beam expander
setting. As the result, a group of calibrated algorithms or look up
tables for different spot sizes with corresponding beam expander
settings is generated.
[0142] 3. Laser beam working distance versus laser beam expander
setting: Use an expander for focus control, zoom expansion control,
or the combination. For instance, a computer controlled embodiment
of the expander 49 shown in FIG. 4, may be used to achieve same
laser beam spot sizes on a work surface at different working
distances. The laser beam focus relative to the work surface could
be held constant or could vary by using different expansion
settings while keeping the same spot size. Then the two-dimensional
calibration described above is repeated for each beam expander
setting. As the result, a group of calibrated algorithms or look up
tables for different working distances with corresponding beam
expander settings is generated.
[0143] 4. Machine vision field of view versus vision lens/camera
setting: Adjust the zoom and focus on vision lens/camera 13, 15 of
sub-system 14 to achieve different sizes of field of view on a work
surface. Repeat and generate a calibration algorithm or look up
table for each vision lens/camera setting. As the result, a group
of calibrated algorithms or look up tables for different fields of
view with corresponding lens/camera settings is generated. On an
alternative arrangement, "software zoom" capability provides for a
useable range of operation without requiring moving parts. In yet
another arrangement the digital and optical techniques may be
combined.
[0144] In a preferred arrangement capability will be provided for
adjustment of system parameters (e.g. laser beam working distance
and spot size) while maintaining calibration in the presence of
"sag" or workpiece warpage. The warpage may be significant relative
to the depth of focus for smaller spot sizes, particularly for
thinner wafers or workpieces (e.g. 300 .mu.m thick, 300 mm
diameter). In one embodiment the alignment vision system 14 (e.g.
positioned relative to the first side) and marker coordinates may
be calibrated with at least the following steps:
Step 1.
Camera Calibration:
[0145] Use a precisely made grid template 91 (shown in FIG. 11-D)
to calibrate the fine alignment camera's pixel size over the field
of view 25 to the real world unit. This will compensate for
geometric distortion of the lens system and other static errors. In
an alternative arrangement a single "point" target may be
translated through the camera field providing stage limited
accuracy performance over the field 25, at the expense of
additional calibration time, but may eliminate a requirement for
the grid.
Step 2.
X-Y Table Calibration:
[0146] Use the fine alignment camera sub-system and x-y stage 18 to
measure a precisely made full field size grid, which approximates
or matches the workpiece dimension (e.g. largest workpiece to be
processed with the system). This step will compensate for static
errors (e.g. tolerance stackup), including non-linearity and
non-orthogonality of the stages.
Step 3.
Marker Field Calibration:
[0147] Laser mark a full field size 24 grid on a mirror 92, as
shown in FIG. 11C. Use the calibrated fine alignment camera (from
step 1) and the calibrated x-y table (from step 2) to measure each
mark 95 of the grid on the mirror over a marking field 24. This
step will compensate for geometric distortion of the laser scanning
lens and Galvanometer system and other static errors.
Step 4.
Three Dimensional Marker Field Calibration:
[0148] In order to compensate for wafer sagging and warpage, the
wafer is marked a plurality of levels along the Z-axis 26. Multiple
marker field calibrations may be required. In this case, relative
motion of one or more of the (1) stage 18, (2) marking head 19 or
internal optical components, for instance expander components 49 of
FIG. 4, (3) stage 28, or (4) chuck 17 provides for relative
positioning of the marking beam and grid. The marking occurs at
several pre-determined levels along the Z-axis 26. Step 3 is
repeated for each level.
Step 5.
Three Dimensional Fine Alignment Camera Calibration:
[0149] In order to compensate for different wafer thickness,
focusing of the fine alignment camera is set at some slightly
different surface levels. The focusing operation may include
translation of the fine alignment sub-system 14 along the Z-axis,
or by adjustment of lens system 15, or in combination. Similarly, a
Z-axis stage may be used to translate the workpiece. Multiple
vision field calibrations may be required. In this case, fine
alignment camera will focus at several pre-determined surface
levels along the Z-axis. Step 1 is then repeated for each surface
level.
[0150] The technique in Step 4 will also allow setting different
spot sizes (by de-focusing) on the fly for different applications
Various curve fitting methods known in the art may be applied at
each of the calibration steps to improve precision. The technique
in Step 5 can also be applied to register the mark inspection
camera 20 and fine alignment sub-system. For instance, the optical
centerline 29 may be approximately aligned at setup and the
calibration procedure used to precisely register the sub-systems.
This is desirable so that the inspected marks may be displayed with
a mark overlaying the corresponding die (for visual inspection),
for instance. Software will be programmed to select correct
calibration files for proper application.
SECTION 2--Feature Detection and Fine Alignment
[0151] In the GSI Lumonics WH4100 wafer marker, offered by the
assignee of the present invention, a fine alignment vision
sub-system corrects rotational or offset errors (X, Y, Angle) which
are introduced when a wafer is placed in the marking station. A
manual "teach tool" allows the user to train the system to
recognize three non-collinear points on the wafer that is to be
used for the correction. The operator selects three regions of the
wafer (e.g. three corners of the overall pattern 115 of FIG. 12.
During 4100 operation a positioner then positions the camera over
the wafer and a die corner is visually selected. A "vision model"
of the region is generated using an iterative trial and error
process with various adjustments. For instance, lighting
adjustments are used to enhance contrast so that an acceptable
match ("model score") is obtained at each of the measurement
locations. Manual evaluation of the results is required with the
system. The model information is then used to determine mark
locations on the bottom side of the wafer.
[0152] The model 4100 is used to process wafers up to 200 mm in
diameter using a "full-field" backside laser marker (e.g.: marker
field covers the entire wafer). However, future generation marking
systems will require marking of wafers up to 300 mm, for example,
with miniature die or packages of finer dimensions (e.g. 0.5 mm).
Also, smaller wafers may also be produced in the future with die
sizes a fraction of a millimeter.
[0153] Referring to FIGS. 1A and 12A, in a preferred embodiment of
a system of the present invention the die pattern layout 115 and
locations for mark registration (e.g. reference data from the first
side) are automatically determined by pattern matching of circuit
features across the wafer 3 using a vision sub-system. Preferably,
no operator intervention is required, or at least the intervention
is substantially reduced. In certain applications the number of
regions to be analyzed may be increased (beyond three) to improve
estimates.
[0154] By way of example, FIG. 12A illustrates several features,
which may be used in the matching process. Within a die 112 circuit
features may include pad 5 which may be an interconnect, but as
illustrated may be a local fiducial. Other features to consider
include trace edge locations 7, die outline 6, or corner 110
locations. As shown in FIG. 1D similar information may be obtained
from a grid array of interconnects, for instance the die edge 6 or
location of the Grid Array ball centers 8. The former approach is
preferable, if the contrast is high. However, if the contrast is
low at the location 6 between the die edge and the surrounding
"street," the grid array locations or other features may be
selected for training (e.g. if higher in contrast). Similarly, the
system may be trained to include the spacing 114 between the die.
It is contemplated that the average measured spacing between
several die (e.g. average pitch) will be a reliable measure and
easy to relate to an available "wafer map." For instance, the
average spacing may be measured between every die and the results
averaged. The available wafer map provides coordinates of the die
within the pattern and associated information for marking. Such
information may be obtained by estimating the locations of die
edges (e.g. least squares fit) near the corners, or with the use of
correlation techniques to match a grey scale or binary image of
region 116, which may be defined from the corner locations. Other
features which may be present include local fiducial(s) 113 (if
present), or identification marks (letters, codes, etc). Such
features may be used alone or in combination with the above.
[0155] Those skilled in the art of machine vision measurement and
pattern recognition will recognize that a number of tools may be
used to obtain the information be used for the automatic teaching
method. For example, the AcuWin vision software provided by Cognex
is suitable for performing various internal "matching" operations.
WO0161275, earlier cited herein, also teaches various automatic
learning algorithms for use in a 3D system for inspection.
[0156] In one embodiment, during the training operation, a wafer is
loaded into the system after the pre-alignment step. The algorithm
then determines at least one of three regions for training based on
wafer map information. The region information will often be
replicated over the wafer, so a single pattern may apply to the
entire wafer. Preferably, the system is calibrated with the 2D/3D
calibration process prior to teaching, but a complete calibration
may not always be required. Referring to FIG. 2A, the wafer 11
(corresponding to 3 of FIG. 1A) and alignment vision system 14 are
relatively positioned to view the region. Feature detection
algorithms are executed, ultimately producing coordinate locations
for the die (and the backside marks). Preferably, the contrast
between the image features is also automatically controlled by
lighting or focus adjustments to improve performance. Methods for
focus and illumination control are well known in machine vision and
non-contact optical metrology. Preferably, the process is repeated
in each region to obtain performance statistics for various
features that may be ranked and selected accordingly for marking
subsequent wafers.
[0157] FIG. 12A shows a view of the of the front side, with a notch
604 (or alternatively, a flat as shown in FIG. 12A) at the bottom
of a typical wafer. In order to generate this transformation for
each wafer at run-time, a minimum of three points that are easy to
locate and span a reasonably large portion of the wafer surface
area are to be selected. In at least one embodiment of the present
invention, a position that can be calculated based on qualitative
information is associated with the point (such as die
corner-upper-left, upper-right, lower-left, or lower-right-and die
row and column number). FIG. 12A shows three exemplary dies
602,601,603 which may be used. The expected location of each point
is calculated based on the information, and may be used to
construct a "theoretical polygon" that is substantially aligned to
the movement of an XYZ Stage. At run-time, prior to processing each
wafer, pattern-recognition software is used to determine the actual
coordinates of these three points on the wafer as it sits in the
nest. These points are used to construct an actual polygon that is
aligned to the die pattern on the wafer. The polygons are then
compared to obtain a transformation (e.g.: translation, rotation
and/or scale) between the two coordinate systems. The table below
contains basic the information that is to be generated for each
point of any given part type before any wafers of that type are
processed by the system. TABLE-US-00002 # Generated Output Data for
Each Point 1 Row and column number of the associated die at that
point. 2 The die corner used; upper-left, upper-right, lower-left,
or lower- right. 3 A vision model of the area around the taught
point. 4 Coordinates of the point in the "primary" coordinate
system.
[0158] The purpose of the FineAlignment training procedure is to
generate this information for a particular part type. The table
below contains preferred information about a part type that is to
be entered into the system before training can begin.
TABLE-US-00003 # Input Data for Each Part Type 1 The number of rows
and columns of actual dies on the wafer. 2 The X and Y pitch of the
dies on the wafer. 3 The X and Y die size. 4 The size of the
wafer.
[0159] Referring to FIGS. 12A-12D, preferably, in order to generate
the information shown in the output data table for each point, any
portion of any die may be positioned at the center of the fine
alignment camera's field of view. The location of the die pattern
115 on the wafer and the orientation of the die pattern coordinate
system 605 relative to the "primary" coordinate system having
origin 607, which is aligned with the movement of the XYZ
Stage.
[0160] Three pieces of information are sought: [0161] a. The
coordinates 610 of a point (x1,y1) in the primary system on the
left edge of the die pattern bounding box 606; [0162] b. The
coordinates 611 of a point (x2,y2) in the primary system on the top
edge of the die pattern bounding box; and [0163] c. The rotation of
the die pattern coordinate system 605 relative to the primary
coordinate system 607.
[0164] With this information the location of the upper-left corner
606 of the die pattern bounding box in the primary coordinate
system may be determined. The origin of the die pattern coordinate
system is then a die_pitch_y up and a die_pitch_x to the left of
that as shown. With the position and orientation of the die pattern
coordinate system known, the stage may be moved relative to any die
location on the wafer.
[0165] Upon determining the locations of two actual die corners
along the left and top edges of the die pattern, and with
capability for positioning any die location in the field of view, a
search is performed (e.g.: search up/down and left/right) from
these two corners looking for the last die in each direction. The
target dies for this algorithm are 602,601,603 in FIGS. 12A, 12B
and FIG. 1A. Each point is then chosen as one of the four corners
of each die. In order to ensure the uniqueness of the area
surrounding each corner, the lower-left corner of die 602, the
upper-left corner of die 601, and the upper-right corner of die 603
would be selected.
[0166] A vision model is to be generated in the area around each
corner (including at least a portion of all four neighboring die
locations). The model may include various features corresponding to
the model of FIG. 12A (e.g.: corners, edges, etc.) The data for all
three points is stored for later retrieval by part type, to be used
at run-time for processing all wafers of that part type.
[0167] Various alternatives may be used to practice a
semi-automatic or automatic training algorithm. For instance,
additional die may be selected throughout the wafer and least
squares fitting done to improve estimates.
[0168] An overall fine alignment process may be semi-automatic, but
with an algorithm for automatic measurement of the die pitch with
enhanced accuracy. By way of example, the process may begin with a
wafer transport tool moving a wafer to the nest. A user interface
and display allows an operator to move a wafer stage 18 of FIG. 11A
(or alternatively a marking head with the wafer held stationary) to
locate a die near the center of the wafer. A pattern, for instance
similar to that shown in FIG. 12C, is selected which will be used
for the alignment process. An image of a wafer portion is displayed
and features identified, for instance the lower corner of a die. A
selected region for "teaching" may be evaluated for automatic
recognition and the lighting adjusted as indicated for the WH 4100
system previously offered by the assignee of the present invention.
Commercially available pattern recognition software may be used,
for instance the Cognex AcuWin vision software.
[0169] In at least one embodiment the die pitch is measured prior
to setting up the at least second and third alignment locations or
the at least three locations 601,602,603 used to transform
coordinates. The operator may position the stage and view the wafer
to identify a suitable row of die and further identify die corners,
for instance the lower left and upper left corner of a die. The
stage may then be moved (e.g.: interactively) to the next die and a
corner location identified from which the die pitch in a first
direction is estimated. The process is then repeated in the
orthogonal direction.
[0170] Preferably the estimate is improved using a program to
obtain additional data by traversing the wafer along rows and
columns, identifying useable die, and locating features (e.g.:
corners) of the die with a pattern recognition algorithm. The data
may be obtained at each row or column where useful data is
available, or in larger increments. The average spacing may be
estimated and related to a wafer map.
[0171] In a present system of the invention "ease of use" and
minimal operator intervention are considered beneficial
improvements. Operator inputs may be valuable to verify a column of
die are useable, for instance. In one embodiment the operator may
verify that a selected die corner is useable and in a "topmost"
column.
[0172] The additional locations for pattern matching are the
selected, the stage positioned, and a test to verify the correct
pattern recognition software operation.
SECTION 3--Workpiece Chuck/Positioner
[0173] It is desirable to grip and hold workpieces of varying
shapes for the application of second side marking based on
first-side data. Similarly, a preferred arrangement can also be
adapted for general "double sided" laser processing and/or
inspection operations.
[0174] Generally, at least one workpiece positioner is provided to
relatively position the workpieces, and configured so as to support
and position workpieces of varying specified dimensions. The
arrangement allows radiation beams to directly irradiate the first
and second sides of the workpiece over a large working area.
Further, damage to the workpiece is avoided which might result from
mounting on a fixture. Still further, a desirable arrangement
allows for a robot driven end effector to load a workpiece without
movement of chuck.
[0175] In at least one embodiment a method and system for edge
chucking and focusing populated and blank silicon wafers of
variable diameters and thickness is used. The method and system may
also be used for other applications, for example in a
micromachining process where a radiation beam is to irradiate both
sides of the workpiece.
[0176] FIGS. 7A-7D illustrates four views of a positioner (top,
end, and side views 7A-C, respectively, and perspective view
7D).
[0177] The "chuck" system includes one or more positioners for
supporting workpieces of varying sizes, and for fine positioning of
the workpiece with one or more degrees of freedom. The chuck system
is mechanically coupled to the X-Y translation stage 18 of FIG. 2A
or other system components. Referring to the side view of FIG. 7C,
a positioner includes a first axis drive 55 (linear stepper motor
illustrated), a horizontal linear drive. It is to be understood
that the drive may be achieved by various methods: e.g. 1. two
position, open loop system such as pneumatic cylinder; 2.
multi-positional, closed loop system such as a linear stepper or
servo and guide. The pneumatically driven method may be the lowest
cost alternative, but provides less positional flexibility. The
first axis drive is used to position a second vertical (or normal)
linear axis (again achievable through various methods) in the
correct location to hold the workpiece. A link 52 between axes
provides the coupling. The second, normal or vertical drive 54 is
used to position the workpiece at the correct height and
orientation (e.g. a plane relative to an X-Y-Z coordinate system)
to be in focus to and irradiated by a marking, inspection, or other
radiation beams. Attached to this second axis drive 54 (rotary
stepper 57 with lead screw and linear guide rail 58 shown) is a
holding or "chucking" mechanism 51. By way of example, the
workpiece clamping mechanism of FIG. 7C is a pneumatic rotary
actuator 51 with clamp arm 59. Alternatively, the arrangement may
be any combination of vacuum and positive mechanical clamping (such
as a pneumatic rotary actuator and a support base). The support
base 53 may optionally have vacuum ports, or a base with vacuum and
no clamping device, for holding the workpiece while it is
positioned and subsequently irradiated or inspected. In FIG. 7C a
workpiece support base 53 is shown without vacuum ports. The
perspective view in FIG. 7D illustrates the shape of the support
base.
[0178] The workpiece positioner (e.g. positioning sub-system) may
be constructed as shown in FIGS. 8A-8D to hold and adjust
rectangular workpiece 61 using two positioners 62, 63 each having
the construction described above.
[0179] A chuck configuration utilizing, but not limited to, nor
requiring, three positioners, driven by closed loop linear steppers
or servos, is the preferred method for holding most workpieces.
FIGS. 9A-9C illustrate an arrangement with three positioners 66,
67, 68, each which may have the construction above, and an
exemplary round workpiece 64, which may be a Silicon wafer (e.g.
100, 200, or 300 mm diameter). The wafer is transferred with end
effector 69 which is a component of a robot loading tool used in a
semiconductor manufacturing process, for example.
[0180] In operation, under control of a computer program, the
workpiece is loaded by adjusting the distance between support 53
with the first axis drive(s) to match the width of the workpiece.
At least the height, and preferably the attitude is controlled with
the additional axis. This generally provides, when used in
combination with other system components, at least five axes of
adjustment (e.g.: X,Y,Z, roll, pitch). Further, the adjustment may
be dynamic and occur during the laser processing operation or
during idle periods.
SECTION 4--Laser Parameters and Mark Quality
[0181] It is desirable to produce high contrast, machine-readable
marks, at high speed in a designated region (e.g. specified by
length, width, and depth). Further, conformance to industry
specifications prohibits damaging or otherwise adversely affecting
the function or operation of the articles (e.g. a semiconductor
device).
[0182] FIG. 10A illustrates an embodiment which can be applied for
various high speed workpiece 77 marking applications. Pulses
generated from a Q-switched Vanadate Laser 71, having a typical
output wavelength of 1.064 .mu.m, are shifted by wavelength shifter
72 to a shorter wavelength for efficiently coupling the energy into
the workpiece. For wafer marking a frequency doubling crystal will
produce a wavelength output at about 532 nm. The optical switch 73,
typically an acousto-optic modulator, is computer controlled to
allow pulses to reach the workpiece 77 on demand. The motion of the
workpiece mounted on stage 79 and X-Y galvanometer deflectors 75 is
coordinated by the computer. U.S. Pat. Nos. 5,998,759 and
6,300,590, assigned to the assignee of the present invention, teach
various aspects related to "pulse on demand" control techniques
using a high speed optical switch as applied to semiconductor
memory repair. Beam positioning accuracy of about 0.3 .mu.m is
typically achieved for cleanly removing semiconductor links.
[0183] Preferably the laser output will be generated from an
Neodymium Vanadate laser with a wavelength of 1064 nm for
processing metal based substrates. The output will be frequency
doubled using the second harmonic generator 72 to be 532 nm for
non-metal substrates (e.g. silicon or gallium arsenide).
[0184] When practicing the present invention various alternatives
may be used for pulse energy control, for instance, controlling
(pulsing) the pump diode power for "marking on demand" with a
series of pulses. U.S. Pat. Nos. 5,854,805, 5,812,569 describe such
methods as applied to workpiece processing. A method of pulse
control in laser systems is also described in U.S. Pat. No.
6,339,604. Various combinations of pump, q-switch, and optical
switch controls may also be of benefit for controlling the energy
output, improving reliability, etc.
[0185] In a preferred embodiment for marking, a telecentric lens 76
and optical sub-system 74 are used to control the spot size and
distribution, which preferably will include optics for varying the
spot-size and focus position under computer control.
[0186] In application to laser marking output pulses are produced
having a set of pre-determined pulse characteristics including a
repetition rate (and corresponding temporal pulse acing), pulse
width, and output energy.
[0187] Selected pulsated by the switch 73 or otherwise controlled
(which may be a "burst" or "string" pulses) irradiate the wafer 77
surface at a first predetermined marking location within the
marking field of the mirrors 75. The stage 79 may be a step and
repeat stage used when the workpiece is larger than the marking
field (e.g. as also illustrated for the "second side" case of FIG.
2A). Referring to FIG. 10B, a laser pulse penetrates the wafer
surface (e.g. silicon) within a depth range sufficient to produce a
machine readable mark 781 at the marking location. Damage to the
wafer is avoided by limiting the depth of penetration 782 (as might
be measured by the l/e energy level) with control of the pulse
characteristics, for instance the peak energy and pulse width.
Deeper penetration 784 results in a crack. In a preferred system
the laser energy at 532 nm will be absorbed at a maximum depth of
10 .mu.m in a typical silicon substrate. This control prevents
micro cracking 783 and other hazardous effects inside the substrate
(e.g. bubbles). The step of irradiating is repeated at a plurality
of marking locations.
[0188] Preferably, the pulse width will be within a range of about
10 to 15 nsec to produce a mark with sufficient contrast.
[0189] The energy per pulse incident on the surface is preferably
in the range of 0.00023 to 0.00025 Joules (eg: 230-250 microjoules)
produces high quality marks on Silicon wafers.
[0190] Preferably, the marking speed is improved to a higher linear
speed on the wafer surface 77, with a relatively high
pre-determined pulse frequency of the laser 71. By way of example,
a repetition rate of about 15-30 KHz, for instance 25 KHz, provides
significant improvement over earlier wafer marking systems used at
both near Infrared and Green wavelengths. With a preferred spot
size of about 30-35 .mu.m, linear marking speed greater than 150
mm/sec is a relative improvement over previous wafer mark systems.
A speed of about of 350 mm/sec is expected for use in a system
having the preferred laser pulse characteristics. Reduced solid
state laser power at high repetition rates constrained earlier
performance, and separation of spots on the surface were observed
which limited mark quality.
[0191] A laser pulse is focused into a spot diameter to produce
energy density within a predetermined range. The minimum distance
between a pair of machine readable marks may be further reduced by
controlling the spot diameter with optics 74. Such an arrangement
may include a "zoom" beam expander in 74 or other optical
components which are removable/insertable, preferably under
computer control (e.g.: as shown in FIGS. 4 and 6). The spot size
adjustment is generally desirable to control the mark linewidth and
contrast. A spot diameter in a range of about 30 to 35 .mu.m and a
working distance to the workpiece of about 220 mm to 250 mm
represent exemplary ranges of operation. The smaller spot size
provides improved capability for producing higher mark density
compared to earlier marking systems, and higher speed is provided
with the pulse characteristics.
[0192] Results for backside marking of Silicon wafers have shown
the depth range of a mark is to be about 3 to about 4.5 .mu.m so to
produce a machine readable mark 781 with enough contrast to the
background. The result was contrary to an expectation that larger
penetration depth was required. The results also provided
additional margin for avoiding damage.
[0193] FIG. 14 shows a top view of a mark 950 to illustrate
measured variation of average marking depth 951 with various laser
parameters. The height in the table below represents material 952
on the side of the mark resulting from removal of material by
melting. The average depth variation measured with an
interferometer illustrates exemplary performance with laser power
and repetition rate at various marking speed. The 100% rating
allows for an estimate of maximum performance. The following data
was obtained: TABLE-US-00004 Average Laser Power Rep. Rate Mark
Speed Average Mark Mark (% max. rating) (KHz) (mm/sec) Height
(.mu.m) Depth (.mu.m) 80 20 120 4.36 -4.75 80 20 200 4.53 -4.43 80
20 300 4.65 -5.61 100 10 120 3.58 -5.40 100 10 200 3.41 -4.33 100
10 300 3.64 -2.90 100 20 120 4.08 -9.91 100 20 200 3.58 -6.45 100
20 300 3.55 -4.53
[0194] Further analysis of the marks indicated sufficient contrast
for machine readability over a range of about 3-4.5 .mu.m.
Increasing the mark depth to the larger numbers, for instance 9.91
.mu.m, produced cracking.
[0195] An interferometric scan was obtained of a wafer marked at a
repetition rate of 10 KHz and a marking speed of about 120 mm/sec.
Severe cracking as exemplified by the "spiky" data which results
from structural variations at a depth of about 9 .mu.m or more.
Scans also showed good results with maximum depth of about 4
.mu.m.
[0196] The shifted wavelength may be below the absorption edge of
the workpiece material, but need not be restricted to 532 nm. For
instance, the workpiece may be Silicon wafers or metal. The
wavelength will preferably be substantially less than the
absorption edge of Silicon (1.12 .mu.m) for marking in accordance
with the present invention.
[0197] Suitable lasers may include commercially available diode
pumped (DPL) Nd:YAG lasers with about 6 Watts IR output, and output
3 Watts in the Green. An alternative, though more expensive, is a
10 Watt (W) DPL laser with about 10 W IR and 5 W green output
power. Preferably, the optical system will contain high efficiency
optical components to minimize losses.
[0198] The Vanadate laser is preferred for marking Silicon wafers,
but is not essential for practicing the invention. The desired
pulse characteristics may be implemented with other designs,
provided all specifications (e.g. beam quality, stability) are met.
For instance, a fiber optic amplified system (e.g. Master
Oscillator Power Amplifier) may be used to produce short pulses at
relatively high rates. A solid state laser, including a fiber
laser, with a slower repetition rate but sufficient power may be
"pulse stretched" with a delay line and beam combiner(s) to
increase the output repetition rate of the laser system.
SECTION 5--Precision Telecentric Lens
[0199] In precision laser marking and other similar material
processing applications, for instance embedded resistor trimming,
there is a need to produce fine spot sizes so as to control the
width and contrast of a mark (or kerf) while maintaining precise
spot placement over a relatively large 3-dimensional field. For
instance, a 300 mm wafer may have die sizes ranging from about one
millimeter or less with a tightly constrained marking region
defined within the die. A spot size of about 30 .mu.m will produce
high contrast marks, but the depth of focus is about four-times
less than that of earlier marking systems. For thin wafers the
warpage may be a significant fraction of the depth of focus, so the
three-dimensional spot size/spot placement considerations are
valuable.
[0200] In non-telecentric scanned laser systems, spot placement
errors at a workpiece plane will vary with depth, and may
significantly degrade the system accuracy. Such z-axis error may be
the result of workpiece tilt, defocus, sag, warp or any deviation
from an ideal target plane. For non-normal incident angles of the
scanned beam, the z coupling is approximately the deviation angle
from normal incidence times the local z error. Preferably, a
telecentric scan lens is used for focusing the marking laser onto
the field. The telecentric scan lens, well known in the field of
laser scanning, is used to maintain a near normal incidence angle
of the beam to the workpiece thereby minimizing z coupling and the
resulting x and y position errors. The approximate invariance of
angle over the field may also have other advantages, such as
providing for coaxial detection of reflected radiation. Coaxial
detection can be used with many know methods to determine focus
position, for example astigmatic spot detection.
[0201] Considering the first order scan lens properties, placing
the scan origin at the front focal plane of the lens will produce a
telecentric scan. In practice, there are non-linearity errors in
the lens design that deviate from perfect telecentricity. Those
skilled in the art of scan lens design recognize that correction of
these errors is possible by modifying the individual lens elements
and/or adding additional elements to the lens design.
[0202] Typically, an x y galvanometer scan system has two scan
mirrors, as shown in FIG. 4. A distance sufficient to prevent
physical interference and beam occlusion separates the mirrors. The
mirror separation creates different scan origins for each axis and
therefore prevents both axes from being located at the lens front
focal plane. Often, the focal plane is placed at an intermediate
position. This creates an additional field dependent telecentricity
error, based on the mirror locations and the lens focal length. In
a typical system the error may be 1 to 2.5 degrees at the worst
field point. Various techniques are useful for correcting
telecentricity error, for instance as described in U.S. Pat. No.
4,685,775 by Goodman, which is hereby incorporated by reference in
its entirety. A beam translator improves the correction.
[0203] With field dependent error, a portion of the field may be
selected to reduce errors at the workpiece. For instance, a small
central portion of the field is used and material is processed with
improved telecentricity. With one scan mirror located near the
front focal plane of the lens, a first axis of the field addressed
with this mirror will have better telecentricity than a second axis
addressed by a second mirror more remote from the front focal
plane. In this case, a portion of the field having improved
telecentricity may be selected with a larger dimension along the
first axis and a smaller dimension in the second axis, for example
a rectangular field. It is also recognized that by using a
rectangular field, the first axis may be larger than the edge a
square field. Selecting a portion of the field may reduce other
field dependent errors such as thermal drift of X-Y galvanometer
deflectors. For example, a quadrant of the field where gain drift
is mitigated in part by offset drift in each galvanometer may be
selected to reduce beam-positioning errors.
[0204] For embodiments using through the lens viewing, the scan
lens is typically required to image a target at wavelengths other
than the processing wavelength. Color correction elements can be
used in a design to improve viewing performance. Telecentric scan
lenses with color correction for through the lens viewing are know,
for instance the scan lens used in the commercially available GSI
Lumonics Model W672 laser trimmer.
[0205] A preferred embodiment for precision laser marking of large
wafers and similar applications includes a three-element
telecentric scan lens 990 as shown in FIG. 13A. This lens has an
effective focal length of 155 mm at 532 nm and is capable of
forming 30 micron spots over a scan field of 80 mm square. The
total path length is about 360 mm. With uncorrected, spaced mirrors
the telecentricity error is approximately 2 degrees. FIGS. 13B and
13C show the telecentricity error 991 and 992 across two orthogonal
scan axes. In both cases the error has non-linear variation. Over a
depth range corresponding to wafer sag of +-300 .mu.m, the worst
case spot placement error is about +-13 .mu.m, slightly less than
one spot diameter.
[0206] In the precision marking system, wherein three dimensional
tolerances determine system performance, the spot placement
accuracy of the lens system is to be maintained by including a
method for three-dimensional calibration. In one embodiment the
wafer is positioned with a workpiece positioner so that a best fit
plane (over the wafer) is aligned normal to the marking head. A
location is then determined relative to best focus position of the
telecentric system of FIG. 13A. The beam positioner is directed
based upon the location of features and stored calibration
data.
[0207] At least one embodiment of the present invention may include
a precision scan lens with improved telecentricity when compared
with a conventional non-telecentric scan lens. In one example, the
maximum angle incident at the workpiece may be less than about half
of the maximum angle of the beam incident on the scan lens entrance
pupil. In another example, the maximum deviation-angle to the
workpiece may be limited to less than about 10 degrees. This type
of scan lens can be smaller, and may be less complex than a larger
telecentric scan lens. Thus, a precision scan lens with improved
telecentricity may be used to provide a design compromise with both
a level of improved marking accuracy with changes in the workpiece
height and reduced lens size, complexity and cost.
Section 6
Backside Mark Visual Inspection with Frontside Die Registration
[0208] In early versions of certain backside wafer marking systems
an infrared source was used to "backlight" a wafer so as to view
backside features. With high density circuitry increasing at a
rapid rate, the "backlight" approach will not always be possible in
the future.
[0209] In one embodiment of a wafer marking system used to form
marks on the backside of a wafer, an inspection feature includes a
registered display of the mark and die. In a preferred embodiment
inspection feature uses two cameras, one above and one below the
wafer. FIG. 2A illustrates the camera 13 of fine alignment vision
system 14 registered along centerline 29 with the mark inspection
system 20. A satisfactory degree of image matching between
corresponding front and backside wafer portions may be achieved
with manual adjustment at system setup, for instance. System
calibration may then be used to improve the precision.
[0210] In at least one embodiment of the system, the equipment
calibrates the bottom camera system 20 to the top camera system.
Preferably, the cameras are in fixed positions. One or more cameras
may have a zoom lens which is manually adjustable. In one
arrangement, a calibration target of a transparent surface is
placed between the two cameras. The image is acquired with both
cameras. The images are superimposed and, using pattern-matching
software, for instance commercially available tools from Cognex
Inc, a correction offset, angle, and scale is calculated to align
the bottom camera's image to the top camera. FIG. 17A illustrates a
calibration target, the image of which is to vary with offset,
scale and rotation. Various other commercially available or custom
targets may be used. The translation, scale, and rotation
correction (including inversion of a coordinate axis) is
automatically determined in software.
[0211] During the inspection operation the top camera is used to
acquire an image of the die on the topside of the wafer. The bottom
camera is used to acquire an image of the mark on the backside of
the wafer. By superimposing the coordinate systems of the two
images, analysis determines the accuracy of the mark with respect
to the die.
[0212] During inspection, this calibration data is applied to the
mark image. Using pattern matching or OCR software the location of
the mark relative to the location of the die is known.
[0213] It is to be understood that this feature is not restricted
to top and backside wafer marking, but may be applied to any two
sides or separated fields.
[0214] Inspection of marks may be done on-line or off-line. The
inspection may include a random sample of die or up to 100%
inspection. In at least one embodiment an operator may setup a
region of interest 900 within a backside image corresponding to at
least a portion of a die as shown in FIG. 17B. Preferably, the
operator will be able to adjust 901 the area of interest, as shown
in FIG. 17C, and make any necessary adjustments from a wafer map or
with minor adjustments between die. A typical mark may occupy
50-60% of the area of a die, but up to about 80% is possible.
[0215] While the best mode for carrying out the invention has been
described in detail, those familiar with the art to which this
invention relates will recognize various alternative designs and
embodiments for practicing the invention as defined by the
following claims.
* * * * *