Determination of best focus for step and repeat projection aligners

A reticle design is provided for step and repeat projection aligners which, when patterned on a wafer (10), allows quick macroscopic evaluation of best focus for used in fabricating integrated circuits. The reticle pattern consists of opaque lines and spaces of widths slightly above the resolution limit of the resist/aligner system. The macroscopic determination of best focus, being macroscopically visible, eliminates the need for microscopic determination, thereby providing a quick and easy method. The best focus determination is made by forming an array (22) of resist fields (14') on the wafer, subjecting each field to a different focus and exposure. Upon development, those fields away from the best focus and at the larger exposure doses will be removed, leaving a parabola (24) of fields whose apex (26) is at high exposure. Best focus is the focus setting used to print the row of fields (B) containing the apex. The procedure may be used at different lens locations and reticle orientations to quantify lens aberrations and field tilt. The method of the invention is also suitably used to qualitatively and quantitatively diagnose and characterize the lens, by printing full fields (34) with varying focus. In this manner, lens astigmatism and curvature and field tilt can be mapped, measured, and understood.

TECHNICAL FIELD 
The present invention relates to step and repeat projection aligners, 
commonly used in processing wafers to fabricate integrated circuits (ICs) 
thereon, and, more particularly, involves the determination of best focus, 
which is employed in wafer processing runs to expose and pattern resist 
films on wafers. 
For lens characterization, relative best focus across a stepper's lens is a 
measure of lens and system problems (field tilt and curvature, and 
astigmatism) that can reduce the focus budget of a photolithography 
process. 
BACKGROUND ART 
A stepper aligner system geometry typically comprises in order a light 
source, aperture blades, a reticle, a lens and finally the wafer. 
Adjustable aperture blades or blinds mask light from passing through parts 
of the reticle and lens so that less than the maximum field can be exposed 
on the wafer. 
"Best focus" is the focus setting used on a step and repeat projection 
aligner that will provide the steepest resist sidewall slope and thus the 
best resolution and linewidth control. The stepper focus setting 
corresponds to the adjustable distance between the wafer surface and the 
reticle/lens. 
In conventional focus determination, one or more sets of resolution targets 
in each of 25 to 80 fields, each exposed at incrementally different focus 
and exposure combinations, are evaluated microscopically (at 200.times. to 
500.times. magnification). The nominal dimension of the smallest resolved 
feature is recorded on a data sheet. The matrix of numbers forms nested 
parabolas (curves of constant dimension) whose low exposure apexes define 
"best focus". Underexposure of positive photoresist provides the several 
parabolas for consideration. The error of this method is estimated to be 
.+-.0.75 .mu.m. A typical time to complete this qualification and 
determine the best focus value is about 25 to 30 minutes. 
A more accurate method of interpretation of such a focus exposure matrix is 
to actually measure critical dimension bars in each field. Best focus is 
found at the center of the smallest bars resolved. However, this method is 
even more time consuming than that described above. 
The foregoing procedure is commonly undertaken at the beginning of a work 
shift, and thus production is held in abeyance pending determination of 
best focus. Also, any changes that occur during the shift that could 
change best focus, such as barometric pressure changes or certain 
equipment maintenance procedures, can require that a redetermination of 
best focus be made. 
For lens characterization, the procedure is similar, except that a maximum 
size field is exposed at different focus settings, and resolution bars are 
read at five or more locations on each field. For each location, the date 
comprising the nominal size of the smallest resolved resolution bars 
versus focus form a parabola whose apex is at best focus for that 
location. The difference in best focus values for different areas is 
caused by field curvature and lens tilt. The difference in best focus 
values for horizontal and vertical lines is due to astigmatism. Both must 
be accommodated in the process focus budget. 
Limitations of the prior art lens characterization method include (1) 
accuracy limited by local resist, underlying film, and exposure variations 
at the site of the resolution bars, (2) operator fatigue limited by the 
number of locations that can be tested, so that a complete lens map can 
only be estimated, and (3) time requirements of the method and the 
replication of the test to verify results. 
A need remains for a method of determining the best focus at one or more 
locations in a more rapid manner. 
DISCLOSURE OF INVENTION 
In accordance with the invention, a method of determining best focus from a 
focus/exposure matrix is provided. The method comprises forming a 
two-dimensional matrix of fields of resist on the surface of a wafer 
comprising varying exposure in one dimension and varying focus in the 
other. Such a patteren is commonly referred to as a focus exposure matrix. 
The resist is then developed, and a parabola is obtained, the apex of which 
is at high exposure. Best focus is the focus setting used to print the row 
containing the apex, or, if the apex is between rows, interpolation is 
used. 
Only those matrix rows near best focus will actually print resolved lines 
and spaces in columns of increasing exposure. Away from best focus, and at 
larger exposure doses, the projected image will have insufficient contrast 
to resolve the lines and spaces, so that all resist will be exposed and 
developed away. After development, the resulting parabola, whose apex is 
at high exposure, is macroscopically defined by the present or absence of 
resist. 
A reticle pattern comprising a plurality of opaque lines and spaces of 
widths slightly wider than the resolution limit of the resist/aligner 
system is also provided in accordance with the invention. 
Resolution in focus choice to 0.25 .mu.m can be readily achieved with a 
matrix of 0.5 .mu.m focus steps. The method of the invention allows quick 
macroscopic evaluation of best focus; the typical time to macroscopically 
choose best focus on a 16.times.16 array is about 30 to 60 seconds. While 
the time required to expose the larger array is about 2.5 minutes longer 
than by the prior art method, a savings in terms of total time (9.5 
minutes versus 25 to 30 minutes for the microscopic approach) more than 
compensates for the additional exposure time. 
Also in accordance with the invention, an extension of the above method, 
using the same reticle pattern for quantifying focus variation across the 
field, is provided. To measure best focus at different locations on the 
field the reticle is masked to print only the area of the field to be 
tested. An over-exposed focus/exposure matrix is printed. The procedure is 
repeated after changing the masking to expose different areas of the lens. 
Each wafer provides a best focus value for that location. The reticle can 
be turned so that best focus can be determined independently for sagittal 
and tangential line. (The difference between these is due to astigmatism.) 
A method of obtaining a global "picture" of relative focus across the whole 
field simultaneously is to unmask the reticle so the whole filed will be 
printed, and print a focus matrix at a constant high exposure. The 
developed wafer(s) will macroscopically display a progression of zones 
that are in and out of focus (as evidenced by the amount of photoresist 
remaining) at different focus settings. An overlay of the fields exposed 
at different focus values contains topographical information about the 
field. This method can be used to identify problem areas for further 
testing with SEM (scanning electron microscope) or other techniques.

BEST MODES FOR CARRYING OUT THE INVENTION 
A. Best Focus 
1. Conventional Method. 
Referring now to the drawings, wherein like numerals of reference designate 
like elements throughout, a wafer 10 is shown in FIG. 1, supporting a 
matrix 12 of resist fields or elements 14 in an 8.times.7 array. The 
matrix of fields has been exposed in a conventional step and repeat 
projection aligner (not shown), with exposure varied in the X-direction 
(horizontal) and focus varied in the Y-direction (vertical) and developed. 
As is conventional, the wafer 10 is "stepped" to permit exposure of a 
field 14 of resist at an adjacent location, employing a different set of 
focus and exposure conditions. 
In this prior art approach, the reticle employed conveniently comprises at 
a number of locations throughout the recticle, a standard resolution bar 
cell 16. A particle example of such a cell 16 is shown in FIG. 2, with 
sets of three nested L-shaped bars 18 having decreasing spacing, for 
example, from 1.3 to 0.7 .mu.m, as indicated. 
An operator examines a set of resolution bars in each field 14 under a 
microscope (500.times.) and finds the smallest set of resolved bars 18 in 
which no resist material remains between the bars. In the example depicted 
in FIG. 2, this is the set at 0.9 .mu.m. 
The operator enters these values on a tabulation matrix 20, illustrated in 
FIG. 3, arranged identical to the array of unique fields of FIG. 1. After 
inspecting and tabulating data from each of the 56 fields 14, a pattern of 
nested parabolas (of constant resolved pattern size) is discerned, with 
each parabola's apex indicating the same best focus. In the example 
depicted in FIG. 3, the best focus is seen to be at 260, as indicated by 
the arrow denoted "A". The operator would set the focus of the column of 
the projection aligner at that value, and processing of wafers would be 
performed at that value throughout the day (except for small offsets 
specific to device and process level), unless the operator suspected that 
focus had shifted for some reason (such as a barometric pressure change or 
equipment maintenance work). Then, the foregoing procedure would be 
undertaken again. 
A typical time to interpret a conventional 8.times.7 focus exposure matrix 
microscopically and extract the best focus value is fifteen to twenty 
minutes. Total processing time (reticle loading, stepping the wafer and 
exposing the resist, developing the resist, and reading the wafer) is 
about 25 to 30 minutes. 
2. The Invention. 
In accordance with the invention, an improved method of determining best 
focus is provided. In this method, an array 22 of elements 14' is formed 
on the wafer 10 by the same step and repeat procedure outlined above, 
varying focus along one direction and exposure along the other. Here, as 
above, exposure is varied along the horizontal direction, while focus is 
varied along the vertical direction. The array depicted is 16.times.16 for 
convenience; other size arrays may be employed, depending on the extend of 
resolution desired. 
The nature of the wafer 10 employed in the practice of the invention is not 
critical. When best focus has been determined, the resist remaining may be 
stripped off with a variety of common methods, and the wafer used again 
for best focus determination or porcessed for forming ICs thereon. 
A layer of resist is applied, conventiently the same resist used in 
production. The resist is conventionally applied by spinning, to a 
thickness ranging from about 8,000 .ANG. to 15,000 .ANG., typically about 
12,000 .ANG.. If the resist is too thick, then it will be difficult to 
resolve small dimensions. Advantageously, the thickness of the resist is 
chosen to be in the same range as that employed in production. 
Next, a reticle is used to pattern the resist, and aperture blades or 
blinds mask the field of the projection aligner down to the desired field 
size. For example, fields of 5 mm square may be employed, with 4 mm steps. 
The smaller the steps, the better the focus resolution attainable, and the 
longer the wafer exposure will take. 
A reticle layout has been developed to be used in the practice of the 
invention. As shown in FIG. 4, the reticle pattern comprises a plurality 
of opaque lines 30, preferably chrome, and spaces 32 of widths slightly 
above the resolution limit of the resist/aligner system. Preferably, the 
widths range from about 1 to 25% above the resolution limit. The width of 
the lines 30 and spaces 32 presently is such as to provide lines on the 
wafer ranging from 1 to 1.2 .mu.m and spacing ranging from about 0.8 to 
1.0 .mu.m. However, as the technology advances and smaller line widths and 
spacing can be printed, the lower limit of line width and spacing may be 
reduced. 
For example, one reticle pattern used compirsed chrome lines 6 .mu.m wide 
and spaced 5 .mu.m apart to print lines approximately 1.1 .mu.m on a 
5.times. stepper (at nominal or typical exposure). The resolution limit 
for the resist/aligner system in this case was about 0.9 .mu.m. The long 
parallel lines extend across a 5 inch quartz reticle so that fields of any 
size up to about 20 mm diameter can be printed on a 5.times. stepper, 
depending upon the application. Of course, for a 1.times. or 10.times. 
stepper, the reticle line size spacing and maximum field size would vary 
accordingly. In any event, the line and space widths depend on the 
resist/aligner capabilities, and simple experimentation will reveal the 
optimum combination for a specific set of conditions. 
Optionally, resolution bar cells, similar to those depicted in FIG. 2, may 
be included for microscopic quantification of resolution. However, such 
bar cells are not needed in the practice of the invention, since no 
microscopic examination is involved in best focus determination. 
Following step-wise exposure, the resist is developed in a conventional 
developer. (If sufficient coarse focus steps and high exposures are used, 
the latent image evident before developing may be adequate to judge best 
focus.) For example, for AZ1512 positive photoresist, an aqueous solution 
of sodium hydroxide is used, as is conventional. Again, the developer is 
keyed to that exployed in production, as is the method of developing (for 
example, spray or dip). 
As a result of the foregoing process steps, a parabolic pattern, similar to 
that depicted at 24 in FIG. 5, emerges. 
The best focus is determined by the presence or absence of resist on 
portions of the wafer, not by measurement or by some subjective assessment 
of "resolution". The method of the invention is seen to exploit focus as a 
parameter which causes macroscopically visible, highly contrasted 
profiles. Thus, the method is referred to herein a the "Macro Focus 
method" of determining best focus. 
The governing concept is that only those matrix rows near best focus will 
actually print resolved lines and spaces in columns of increasing 
exposure. Exposures may range from nominal to several times nominal. 
Away from best focus, and at larger exposure doses, the projected image 
will have insufficient contrast to resolve the lines and spaces, so all 
resist will be exposed and developed away. After development, the 
resulting parabola 24, whose apex 26 is at high exposure, is 
macroscopically defined by the presence or absence of resist, as shown in 
FIG. 5. Best focus is the focus setting used to print the row containing 
the apex, as indicated by the arrow denoted "B". If the apex is between 
rows, interpolation is used. Resolution in focus choice to 0.25 .mu.m is 
readily achievable with a matrix of 0.5 .mu.m focus steps. 
The particular focus range selected depends upon the accuracy required for 
the process. The particular exposure range selected depends upon the size 
of the lines and spaces and the develop parameters. The closer the 
line/space size is to the minimum printable, the less over-exposure will 
be necessary to cause the parabola 24 to end in a peak 26. 
The typical time to macroscopically choose best focus on a 16.times.16 
array is 30 to 60 seconds. The total processing time is about 9.5 minutes, 
or about 1/3 the total time to determine best focus microscopically. 
The method of the invention provides several advantages in addition to 
speed. First, no microscope is required, thus eliminating a tedious task. 
Second, since the method is less subjective, training of personnel is 
easier. Third, the method described herein permits increased resolution in 
foucs determination; that is, smaller focus steps can be readily employed 
without an accompanying increased evalution time or effort. Fourth, data 
suggests improved accuracy over the conventional microscopic method due to 
the larger area under consideration and the consequently reduced 
sensitivity to local resist thickness and film variations, contamination, 
etc. 
Additional interpretation can be placed on the appearance of the printed 
matrix. Particles on the back of the wafer produce a dramatic bullseye 
pattern across several adjacent fields. Chuck or reticle tilt or a 
non-flat wafer is evidenced by a jagged pattern of partial field clearing. 
Autofocus inconsistency due to system problems or wafer irregrularities, 
such as laser scribed identification, show up as fields which do not show 
a logical similarity to neighboring fields. 
B. Lens Characterization 
1. Partial Field--Quantitative. 
In further accordance with the invention, the foregoing methods described 
for use in determining best focus can be extended to allow 
characterization of other areas of the field than the center, which is 
typically used to determine "best focus". 
Wafer type and processing are the same as above, except in the manner of 
use of the reticle described above. 
To measure field curvature and tilt, the reticle is masked so that only a 
portion of the lens will be used. A focus exposure matrix is exposed as 
above and best focus chosen for that area. Subsequently, different 
protions of the lens are unmasked and the procedure repeated, in a minimum 
of five lens locations total, each on a separate wafer. Best focus is thus 
determined for different parts of the lens. The magnitude of the tilt is 
extracted by looking for linear variations in the best focus values across 
the lens. The magnitude of the field curvature is extracted by looking for 
second order variations, after the linear components are removed, in the 
best focus values across the lens. 
To measure astigmatism at any location, the above procedure is supplemented 
by using the reticle in two orthogonal orientations (for example, 
horizontal and vertical) so that individual best focus values are obtained 
for sagittal and tangential lines. The difference at each location between 
best focus for sagittal and tangential lines is the magnitude of the 
astigmatism there. 
It is known that the larger the field, the larger the through-put of wafers 
in production. However, the larger the field, the more area of the lens is 
being used, and thus the greater the likelihood that areas of the lens 
having significant deviations from ideal flat focus characteristics will 
be used for printing of critical circuit features. The foregoing analysis 
can determine if and where such lens problems exist, so that steps can be 
taken to correct such problems as field tilt, and partially compensate or 
avoid areas having aberrations that cannot be corrected, such as field 
curvature or astigmatism. 
2. Full Field--Quantitative/Qualitative. 
Also in accordance with the invention, the full field may be simultaneously 
tested on one wafer per desired reticle orientation, by varying focus but 
keeping exposure at a constant value. This process is equivalent to taking 
a vertical slice (constant exposure column) through the parabola used for 
best focus determination. 
Wafer type and processing are the same as above, except in the manner of 
the use of the reticle described above. 
To print the maximum field through the whole available lens area, the 
aperture blades are adjusted to unmask the reticle. On a 4-inch wafer at 
5.times., about 9 full fields 34 (20 mm diameter) can conveniently be 
exposed, as shown in FIGS. 6 and 7. After developing the wafer, a 
progression of views will be apparent, wherein zones that are near focus 
will have resist, and zones that are out of focus will not (given an 
appropriate exposure that will make this contrast evident over the focus 
range tested). Bright color fringes visible on the wafer strongly 
accentuate lens focal plane distortions. An example of such distortions is 
depicted in FIG. 6, where stippling is employed to represent the color 
fringes. 
The sequence of fields, if overlayed, would yeild a topographical map of 
focus across the lens. If astigmatism is present, wafers exposed with the 
reticle's lines running one direction will show a different pattern than 
wafers processed identically except that the reticle was turned so that 
the lines ran in the orthogonal direction. Exposing two wafers thus 
isolates and accentuates the astigmatism. (Exposing one wafer with two 
half-time orthogonal exposures of the lined reticle described above, or 
exposing one wafer with a reticle having both directions of lines 
overlayed on it would yield a wafer displaying the average best focus, so 
that astigmatism could not be determined.) 
The presence or absence of resist and bright color fringes can be used to 
extract quantitative field distortion in several minutes, compared to 
hour(s) (the time required by the conventional method depends on the 
number of locations tested). 
In the ideal case, with flat lens characterization, the amount of resist 
would be uniform across the field 34 at any given focus setting and no 
fringes would be evident because the whole field is in focus at the same 
focus setting. In subsequent fields, as the focus goes further from ideal, 
the amount of resist should decrease uniformly, with a fading appearance. 
Best focus is in the field where the most resist remains, as indicated by 
the highest concentration of stippling in FIG. 7 or the center of the 
fields having equivalent amounts of resist. 
INDUSTRIAL APPLICABILITY 
The method of the invention is useful in establishing best focus in step 
and repeat projection aligners used to process wafers for fabricating 
integrated circuits and in characterizing a lens in such aligners. 
Thus, there has been disclosed a method for determination of best focus and 
relative best focus for step and repeat projection aligners. A reticle 
design has been provided for use in the practice of the invention. Many 
changes and modifications will be apparent to those of ordinary skill in 
the art and may be made without deviating from the spirit and scope of the 
invention, and all such changes and modifications are considered to be 
within the scope of the invention as defined by the appended claims.