Position location in surface scanning using interval timing between scan marks on test wafers

A particle imager and method for imaging particles on surfaces of substrates. A surface is raster scanned by a collimated light beam and particles on the surface are detected by the scattered light caused by the particles. During a scan path the intensity of the scattered light is measured forming intensity traces and location addresses for the detected particles. Data from each scan path is stored in memory. The imager is pre-calibrated with a test wafer having light scattering marker points spaced at known positions thereon. Scanning the test wafer, a clock measures time elapsed from a start position to each marker point. The corresponding elapsed times and known address locations are stored in memory for reference during data collection.

TECHNICAL FIELD 
The present invention relates to the scanning of surfaces with a light 
beam, and in particular to the determination of the position of the beam 
on a surface during a scan. 
BACKGROUND ART 
In scanning generally flat surfaces for flaws, defects, particles and the 
like, a scanning beam originates from an apparent spot source, such as a 
reciprocating mirror or rotating polygon, i.e. an optical scanner. The 
scanner directs a scanning beam in an arc, but the surface prevents the 
scanning beam from traversing a true arc-like trajectory. If the beam had 
traversed a true arc, the determination of the beam position would be a 
simple matter. But when the beam traverses a plane the beam appears to 
travel faster at distances further away from the scan center. In many 
applications it is important to know the precise beam position. For 
example, in particle detection, particle position on a surface may be 
found only if the beam position is known. In the past, beam position could 
be estimated by knowing the position of the scanner and then calculating 
where beam should be on a surface. However, such calculations usually do 
not take into account factors such as wear on the scanner which cause 
errors relative to a theoretical scan path. 
In the prior art, others have realized that scanner error creates a problem 
which must be corrected for precise particle or flaw position 
determination. For example, in U.S. Pat. No. 4,404,596 Juergensen et al. 
correct positional error due to uneven surfaces of a rotating polygonal 
mirror. While many of the prior art approaches have proved to be quite 
valuable, there is an ever greater need for precision, especially in 
locating dirt particles in ultraclea surfaces, such as semiconductor 
wafers. In wafer inspection, non-imaging particle detectors have been 
invented which accurately signal the presence of micron size particles and 
smaller. Mapping the location of such particles is needed to be able to 
predict whether circuits built on such a substrate will fail due to 
particle presence in a particular location. 
An object of the invention is to increase the precision by which the 
location of a scanning beam on a surface may be determined. 
SUMMARY OF THE INVENTION 
The above object has been achieved in a beam scanner by clocking the beam 
relative to a known starting position and establishing known beam 
locations for subsequent clock intervals. A test wafer is manufactured 
with a plurality of light scattering elements disposed along a beam scan 
path. The scattering elements are very small pits or bumps spaced at 
equidistant positions along the scan path. A plurality of such paths are 
defined across the wafer surface so that at various scan paths across the 
wafer surface, the beam position may be clocked. The known starting 
location is a fixed pin or depression which gives a strong light 
scattering signal. A second pin may be placed opposite the first on the 
other side of the scan path for bidirectional scanning. 
Once a strong light scattering signal is received from the pin marking the 
beginning of a scan, a high frequency clock is started. Each time one of 
the equidistant light scattering elements is encountered in the scan path, 
as indicated by a light scattering signal, the clock pulse count is stored 
at an address in a computer memory. Memory addresses correspond to 
physical locations of the light scattering elements. In other words, the 
computer memory contains addresses corresponding to each light scattering 
element position. With each address is associated a number indicative of 
the clock pulse count from the beam scan starting position. 
Once the memory has been loaded with data from the test wafer, a target 
wafer is substituted for the test wafer. The scanning beam must pass over 
the start of scan marker in order to initiate the clock. Then, as clock 
pulses are counted, those counts, which correspond to the previously 
measured counts, signal the arrival of the beam at a position 
corresponding to the equidistant light scattering elements on the test 
wafer. Particles observed between light scattering elements may be located 
by interpolation. 
After a number of hours of scanner operation, the test wafer is again used 
to generate a set of clock pulse data to be loaded into memory. In this 
way, even very small amounts of wear on scanner mechanical parts will not 
effect the precise determination of beam position location. In 
bidirectional scanning, the same procedure is used in each scanning 
direction, both with a test wafer and with a target wafer. A different set 
of clock data is generated for each scan direction.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to FIG. 1, laser 11, a low power helium-neon or argon ion 
laser generates a beam which is prefocused by optical elements 13, 
typically one or more lenses, to a point beyond the spherical mirror 15, 
namely the surface 17 of a wafer 19 being inspected. After passing through 
optics 13, the laser beam impinges upon a small fixed mirror 21 which 
folds the light path and directs the beam toward scanning mirror 23. The 
scanning mirror 23 is supported on an arm connected to a motor which rocks 
the mirror at its natural frequency of vibration. Such mirrors are known 
as resonant scanning mirrors and the natural frequency of vibration is 
specified by the manufacturer. 
The scanning mirror is aligned at a slight tilt relative to the incident 
beam so that the scanning beam describes a shallow cone in space, but will 
follow a straight line path after reflection from the spherical mirror 15. 
The purpose of the spherical mirror's curvature is to cancel the effective 
field curvature of the prefocused beam to produce an essentially planar 
image field at surface 17. The scanning mirror 23 is optically flat and 
its axis of rotation is not perpendicular to the incoming beam in order to 
generate the shallow cone in the reflected beam, previously mentioned. 
This optical arrangement permits generation of a 100 micrometer scan spot 
on a flat image field with a path straightness within 10 micrometers over 
a total scan distance of 200 millimeters. The scan is nearly telecentric. 
A system with a focal length of about 500 millimeters should scan a 200 
millimeter path. At a wavelength of 488 nanometrs and a final spot size of 
100 micrometers diameter, where the beam diameter is measured at 
1/e.sup.2, the beam diameter would be 16.3 millimeters at the scan mirror 
23. While some aberration may be expected, the maximum aberration blur 
diameter is under 10 micrometers for astigmatism and under 4 micrometers 
of tangential coma, small enough relative to a 100 micrometer spot 
diameter to reduce the spot center intensity only a few percent. The 
estimated deviation from true telecentric scanning is from 1 milliradian 
to less than 40 milliradians. 
With reference to FIG. 2, a telecentric input beam 31 is seen passing into 
an internally reflecting elliptical cylinder 33 through a linear slot 35 
whose length is parallel to the scan direction, i.e. in a plane 
perpendicular to the paper of the drawing. The narrow slit 35 allows 
egress of light specularly reflected from the reflective surface 17 of 
wafer 19. Beam 31 impinges along a focal line 37 extending into and out of 
the plane of the paper of the drawing. Focal line 37 is one of two foci of 
the elliptical cylinder 33. The second focal line 39 is a line where input 
ends of fiber optic fibers 41 are aligned. Thus, any light which is 
scattered from a dirt particle or flaw along the scanning line 37 will be 
reflected to the second scan line 39 and be input into fibers 41. Since 
the scattering is coming from irregular surfaces, the light appearing line 
39 does not form a true optical image of the particle or flaw. Rather, the 
light along line 39 is representative of the scattered intensity from the 
particle or flaw. If the particle or flaw is large, more light will be 
scattered than if the particle is small. Light entering the fibers 41 is 
transmitted to a detector 43 which may be a photomultiplier tube. After a 
wafer is scanned along a line, the wafer is advanced slightly by wheels 45 
or by another support mechanism. By slightly advancing the wafer, another 
line may be scanned. By scanning different lines which are parallel and 
slightly spaced apart, an entire wafer may be scanned. Differences between 
small particles and flaws such as cracks or spurious signals such as noise 
may be interpreted in accord with the particle detection method set forth 
in U.S. Pat. No. 4,766,324 to S. Saadat et al., assigned to the assignee 
of the present invention. 
With reference to FIG. 2A, a wafer 19 is shown with a plurality of sampling 
points at which the scattered signal is sampled on the wafer. The wafer is 
placed between light marker pins 53 and 55 at opposite sides of the wafer 
and aligned with the scanning line of the optical system. As beam 57 is 
swept across the wafer, the amplitude of the scattered signal is sampled 
at specific positions 57a, 57b, 57c so as to cover the wafer with a 
regular array of sampling pints 51 which are preferably spaced a uniform 
distance apart. The start of the array is referenced to the marker pins 53 
and 55. 
The start of the array is timed to coincide with the transit of the beam 
across one of the marker pins (say 53) as the beam moves toward the other 
marker pin 55. The start signal is produced by a light detector placed 
behind either marker which senses the transit of the beam across the 
timing marker. When light from the beam is first received at the detector 
behind marker 53, as the beam passes over the edge of pin 53, it initiates 
a counter which counts pulses from an accurate 50 megahertz clock. 
The output of the counter is continuously compared to a series of 
predetermined values which are stored in a random access memory. When the 
counter value equals the stored value, a pulse is issued to the sampling 
circuit which samples the instantaneous amplitude of the scatter signal 
received at the detector. The predetermined values which are compared to 
the counter output are selected so that the scatter signal is sampled at 
precisely equal distances apart or known positions on the wafer under 
test. The separation between sampling points is approximately 26 microns. 
Any errors in position of the sampling points can be corrected by 
calibrating the position of the beam with respect to the marker pins using 
a standard wafer having scattering sources whose relative positions are 
known to very high accuracy. The apparent position of these scattering 
sources is measured using the initial table of sampling points. The 
difference between the apparent positions and the known positions of the 
scattering sources is used to generate an error function which is used to 
modify the predetermined values which are stored in the random access 
memory. This generates a new table of predetermined values which corrects 
for irregularities in the motion of the beam and defines the position of 
the sampling points to the same accuracy as the reference scattering 
sources on the standard wafer. 
By this means one is able to establish a very accurate coordinate system in 
the X axis where the exact position of a scattering source on a wafer 
under test is known simply by counting the number of sample points from 
the start of the marker pin 53 or 55. 
The Y position is established by counting the number of sweeps from the 
start of the wafer. This establishes an orthogonal set of XY coordinates 
which allows one to access and store data from any point of the wafer and 
store a microscan of a small section of the wafer in that area, without 
incurring any distortion of the stored image. 
With reference to FIG. 3, a wafer 19 is seen having an imaginary grid 
pattern thereon. Lines of the grid parallel to the line A represent scan 
lines. Actual scan lines are spaced by 10 microns, with the beam scanning 
in both directions. The wafer is advanced in the direction indicated by 
the arrow B by a wafer transport so that one scan line after another 
traverses the wafer surface. Thus, the scan mirror 23 achieves scanning in 
the direction indicated by arrow A while a wafer transport provides motion 
to the wafer so that the wafer is also scanned in the direction indicated 
by arrow B. The grid pattern of FIG. 3 has no physical meaning apart from 
showing scan direction. 
FIG. 4 shows an array of equidistant sample points 51 on a small portion of 
a standard wafer. The center-to-center spacing of the points is 26 
microns, while the scan line to scan line spacing is 10 microns. For 
greater throughput, the scan spacing may be increased, with some loss of 
resolution. 
By timing the beam traverses between pins, the scanner amplitude may be 
adjusted to provide for a constant time of traverse between the pins. The 
scanner is tolerant to some amount of variation in its natural frequency 
of vibration, usually up to 2%. This is sufficient to allow precise 
adjustment of the scan time so that constant times are obtained between 
pins. 
The signal going to detector 43 may be adjusted to correct for variations 
in the laser output power. This may be done by calibrating the detector 
for a number of decades of dynamic range with respect to scattering from a 
marker pin. For example, the three top decades of dynamic range for 
photomultiplier tube gain may be recorded. To record further levels, the 
incoming beam may be attenuated by a known amount using a neutral density 
filter. Such a filter could absorb most of the scattered light from a 
light scattering element. The amount absorbed would be set so that the 
detected signal is below the lowest decade of dynamic range measured 
without the filter. Thus, another few levels of dynamic range of the 
detector may be established, allowing the measurement of very small 
signals. 
The preparation of the light scattering elements in a test wafer is shown 
with reference to FIG. 5. Wafer 19 is coated with a thin layer of 
photoresist material 61. The wafer is patterned with a mask with openings 
formed at regions 51 by removal of material by photolithographic 
techniques. The square 63 represents material removed from a square 
opening, creating a light scattering element. As the scanning beam moves 
across the surface of material 61, it encounters the opening and the 
nonuniformity in the surface causes scattering. This technique is 
described further in U.S. Pat. No. 4,512,659 assigned to the assignee of 
the present invention.