FIELD OF THE INVENTION
The present invention is in the field of automatic optical inspection
techniques, and relates to a system for inspecting reticles or masks in a manner
to simulate the operation of a specific photolithography tool in which this reticle
is to be used.
BACKGROUND OF THE INVENTION
Photolithography is one of the principle processes in the manufacture
of semiconductor devices, and consists of patterning the wafer's surface in accordance
with the circuit design of the semiconductor devices to be produced. More specifically,
a circuit design to be fabricated on the wafer is first patterned on a mask or reticle
(for simplicity, the terms mask and reticle will be used here interchangeably, although
in actuality they refer to somewhat different techniques). The wafer is coated with
a photoresist material, and is then placed in a photolithography tool to be exposed
to light passing through the reticle to produce a latent image of the reticle on
the photoresist material. Thereafter, the exposed photoresist material is developed
to produce the image of the mask on the wafer. After the completion of the photolithography
process, the uppermost layer of the wafer is etched, a new layer is deposited, and
the photolithography and etching operations are started again. In this repetitive
manner, a multi-layer semiconductor wafer is produced.
As is well known, photolithography tools utilize a lamp or a laser
as a light source, and utilize a relatively high numerical aperture (NA) objective
to achieve a relatively high resolution. The optics of such tools are generally
designed to produce reduction (negative magnification) of the image of the reticle,
e.g., 1/5 onto the wafer. Different models use different NA and magnification combination,
as designed by the manufacturer of the tool.
It should be appreciated that in order to obtain operating semiconductor
devices, the reticle must be defect free. Moreover, in most modern processes methods,
the reticle is used in a repeated manor to create many dies on the wafer. Thus,
any defect on the reticle will be repeated multiple times on the wafer and will
cause multiple devices to be defective. Therefore, various reticle inspection tools
have been developed and are available commerically. One type of such inspection
systems, to which this invention pertains, scans the entire reticle using an illumination
spot technique to inspect the reticle for defects. Examples of such systems are
provided in USP 4, 926,489, 5,838,433, and 5,563,702, and is also schematically
depicted in Figure 1.
As shown in Figure 1, a reticle 10 is placed on an x-y
stage 20. A laser 30 produces an illumination beam of a relatively
narrow diameter. A scanner 40, e.g., a rotating mirror or an acousto-optic
deflector (AOD), is used to scan the beam in one direction, generally referred to
as the "fast scan" direction. The stage 20 is moved in a direction perpendicular
to the fast scan direction in a serpentine manner, so that the entire surface of
the reticle is scanned. The scanned beam passes through the dichroic mirror
50 and is focused by objective lens 60 onto the reticle. Light transmitted
through the reticle 10 is collected by the objective lens 70 and focused
onto light sensor 80, e.g., a photo-multiplier tube (PMT). Reflected light is deflected
by the dichroic mirror 50 to be collected by the lens 95 and focused onto the light
sensor 90. Shown by a dotted line is an optional optics and tilted mirror assembly
that can be used to obtain an interferometer image of the reticle for inspection
of phase shift designs (see, e.g., the cited USP '702).
Conceptually, the inspection systems exemplified in Figure 1 generate
a highly magnified image of the reticle. Each pixel in the image corresponds to
a sampled illuminated spot on the reticle, and has a grey level corresponding to
the amount of light received by the light sensor. This grey level can be either
compared to a corresponding pixel from an adjacent die on the reticle, or binarized
and compared to a database or compared to a gray scale image calculated from the
database. When a discrepency above a designated threshold is encounterred, the location
is identified as suspected of having a defect.
Recent advancements in photolithography technology have introduced
another factor which may cause the latent image on the wafer to be defective. Specifically,
the reduction in design rules necessitates various measures to counter changes in
the latent image caused by the interaction of the light with the design on the reticle.
Such interactions are generally referred to as "optical proximity effects", and
results in, for example, corner rounding, a difference between isolated and semi-isolated
or dense patterns, a lack of CD linearity, etc. Whilst not being detected as potential
defects in a particular reticle by the conventional inspection system, these effects
could produce real defects on the wafer. On the other hand, these effects should
not cause the system to issue an alarm if they will not be transferred as defects
onto the wafer. Moreover, there is a need to inspect the countermeasures, such as
optical proximity conection (OPC) and phase shift etching on reticles, and to test
their design and effectiveness.
Conventionally, in designing and evaluating reticles, especially advanced
reticles having OPC and phase shift features, one has to create the reticle, expose
a wafer using the reticle, and check that the features of the reticles have been
transferred to the wafer according to the design. Any variations in the final features
from the intended design necessitate modifying the design, creating a new reticle,
and exposing a new wafer. Needless to say, such process is expensive, tedious, and
time consuming. In order to short-cut this process, and assist in design and evaluation
of advanced reticles, IBM has recently developed a microscope called the Aerial
Image Measurement System (AIMS).
The AIMS system is disclosed, for example, in European Patent Publication
No. 0628806, and in the following articles: Richard A. Ferguson et al. "Application
of an Aerial Image Measurement System to Mask Fabrication and Analysis", SPIE Vol.
2087 Photomask Technology and Management (1993) pp. 131-144, and R. Martino
et al. "Application of the Aerial Image Measurement System (AIMS™) to the
Analysis of Binary Mask Imaging and Resolution Enhancement Techniques",
SPIE Vol. 2197 pp. 573-584. The Microscope is available commercially
from Carl Zeiss, GmbH of Germany, under the trade name MSM100 (standing for Microlithographhy
Conceptually, rather than obtaining a highly magnified image of the
reticle, as is done by inspection systems, the AIMS system emulates a stepper and
creates a highly magnified image of the latent image produced by the reticle. Specifically,
the operational parameters of illumination and light collection in the AIMS, such
as wavelength and NA, can be adjusted by the user to simulate the tool which will
be used to expose wafers using the reticle. The illumination is provided in a manner
which simulates exposure in a stepper, so that a latent image of the reticle is
created. However, rather than placing a wafer at the location of the latent image,
a sensor is places so as to produce an aerial Image of the latent image produced
by the retiole. Also, rather than providing reduction of the image like a stepper,
the AIMS magnifies the latent image to enable easier image aquisition
The AIMS is basically an engineering tool, which is intended for development
and testing of various reticle designs. It is also helpful for checking how OPC
and phase shift features would print on the wafer. Additionally, the system can
be used to study various defects discovered by a reticle inspection systems, and
test whether those defects would actually print on the wafer. However, the MSM100
is not intended to be used as a general reticle inspection system, and lacks any
of the technology required for rapid inspection of reticles.
U.S. Patent 5,795,688, however, discloses a technique for using a
system such as the MSM100 to perform an automatic inspection of a photomask. To
this end, an aerial image of a portion of the photomask is acquired with the MSM100,
while a so-called "virtual stepper" software algorithm concurrently simulates a
similar aerial image considering the operational conditions of a specific stepper
of interest, using the reticle pattern data base. The real aerial image is compared
to the simulated aerial image, and potential defects on the photomask are located.
This technique actually utilizes a so-called die-to-database image processing technique,
wherein the database is constituted by the simulated image. Since the image is obtained
using the MSM100, which cannot perform rapid inspection, this technique cannot be
used for in-line automatic inspection of reticles progressing on a production line.
On the other hand, this technique does not provide reliable results due to limitaions
of the simulation software. Specifically, many artificial differences between the
real aerial image and the simulated aerial imago would be falsely flagged as defects.
Accordingly, there is a need in the art for a reticle inspection system
which would be capable of "conventional" reticle inspection in conjunction with
Aerial image inspection. Moreover, the system would preferably be also capable of
detecting particles on the reticle.
The present invention intends to overcome the above problems. The
object is solved by the system for optic inspection according to independent claim
Further advantages, features, aspects and details of the invention
are evident from the dependent claims, the description and the accompanying drawings.
The present invention provides the advantages of automatic optical
inspection of reticles utilizing the laser spot illumination, incorporating a novel
optical inspection method and system simulating the operation of a specific stepper
and specific resist.
It is a feature of the present invention that it can be constructed
by easily modifying any conventional inspection system utilizing a flying spot scanning
of the reticle under inspection.
The present invention utilizes the capabilities of conventional inspection
systems to provide inpection using high resolution imaging of the reticle. Additionally,
the inventive system is capable of inspecting the reticle using aerial imaging.
According to an embodiment the invention a system for automatic optical
inspection of a reticle to be used in a selected photolithography exposure tool
operating with a selected frequency of light and a selected numerical aperture and
coherence of the light, and using a selected type of resist comprises:
- a light source for providing a light beam;
- a scanning apparatus for receiving the light beam and scanning the light beam
to form a flying spot over the reticle;
- an objective optics having a defined numerical aperture for high resolution
- a detection unit comprising a light sensor for receiving light transmitted through
the reticle and generating data representative thereof; and
- a processor unit coupled to said light sensor to be responsive to said data
for analysing it and generating data indicative of defects on the reticle; the system
being characterized in that it further comprises
- an illumination assembly selectively insertable in the path of the incident
light beam and operative to adjust said defined numerical aperture to simulate said
selected numerical aperture of the exposure tool; and
- a collection assembly for adjusting a collection numerical aperture of the transmitted
According to another embodiment the above illumination assembly or
a second aperture is inserted into the beam's path to emulate the effects of the
photoresist in the lithographic process.
According to another embodiment the shape of the illumination beam
is modified from a Gaussian to a flat-top shape.
According to yet another embodiment the system is made to no out of
focus or move some optical elements from their previous location to effectively
expand the beam on the reticle.
A rotating scattering disk may be inserted at a plane where the optical
beam has a very small instantaneous diameter so that it shall reduce the time and
spatial coherence of the beam on the reticle.
According to another embodiment the illumination assembly comprises
an aperture which has also the properties of a beam shaper, such that the profile
of the incident beam may be changed from a Gaussian to such as a flat-top. The collection
channel also includes a collection assembly accommodated in front of the light sensor
for adjusting the numerical aperture of the collection objective. In other words,
by appropriately selecting the illumination and collection apertures, coherence
of light in the inspection system can be adjusted to that of a selected exposure
Preferably, the analysis of the data representative of the at least
transmitted light components includes the comparison of data representative of at
least some of the successively scanned features on the reticle to each other. This
is the so-called "die-to-die" signal processing technique.
The illumination assembly may comprise an illumination aperture, e.g.
an off-axis aperture, e.g. quadrupole. The illumination aperture may comprise beam-shaping
properties, being, for example, a diffractive optical element providing a flat-top
beam profile. Preferably, the aperture reduces the numerical aperture of the illumination
by about four folds, thereby simulating the lower numerical aperture illumination
of a stepper.
Preferably, the illumination assembly comprises a set of different
apertures. Accordingly, the system can resemble the operation of a different stepper
of interest by selecting the illumination aperture type. The collection assembly
preferably includes a light collection aperture, which preferably provides the collection
numerical apature of for example 1.2-0.2.
The detection unit comprises at least one light sensor accommodated
in the optical path of light transmitted through the reticle, which is preferably
a photomultiplier tube (PMT).
The system may additionally utilize a dark-field inspection. To this
end, the detection unit comprises at least one additional light sensor accommodated
so as to collect light scattered from the illuminated spot on the reticle.
Additionally, to speed up the inspection, the system may utilize a
so-called multispot scanning technique. For this purpose, the scanning apparatus
also includes a beam splitter means for splitting the primary laser beam into at
least two beams, thereby providing at least one additional scanning beam. In this
case, the detection unit comprises at least one additional light sensor for receiving
light components transmitted through a spot illuminated by the additional beam and
a lens to separate the two beams to the two light sensors.
More specifically, the present invention is used for inspecting the
reticles used for patterning wafers during the photolithography process, and is
therefore described below with respect to this application. It should be appreciated
that the terms "reticle" and "mask" are used herein interchangiably.
In order to understand the invention and to see how it may be carried
out in practice, a preferred embodiment will now be described, by way of non-limiting
example only, with reference to the accompanying drawings, in which:
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
- Fig. 1 schematically illustrates a system according to the prior art.
- Fig. 2 schematically illustrates the main components of an optical inspection
system according to one embodiment of the invention;
- Fig. 3a illustrates a set of different illumination apertures suitable
to be used in the system of Fig. 2;
- Fig. 3b graphically illustrates the main principles of an apodization
aperture affecting the profile of a laser beam suitable to be used in the system
of Fig. 2;
- Fig. 4 schematically illustrates the operation ofthe system of
Fig. 2, and
- Fig. 5 illustrates the main components of an optical inspection system
according to another embodiment of the invention.
Fig. 2 depicts an exemplary optical inspection system,
200, according to an embodiment of the present invention. The embodiment
of Figure 2 retains many of the elements of the system of Figure 1 and, therefore,
similar elements are designated by the same character preceded by numeral "2." Due
to the special construction of this embodiment, the system of Figure 2 can be operated
in at least two modes: conventional inspection mode and aerial imaging mode. In
the conventional inspection mode the same elements as in Figure 1 are employed to
perform conventional inspection, i.e., using a flying spot to obtained a high resolution
transmission image of the reticle and compare the image to a database or perform
a die-to-die comparison.
As is known and understood from Figures 1 and 2, in a "flying spot"
system the illumination optics has high NA, high resolution characteristics, so
that a small spot is illuminated on the reticle. Then, a sensor, such as a PMT collects
all the light it receives and is sampled periodically, The size of the spot and
the sampling time determines the pizel size and resolution. This is in contrast
to imaging optics, such as in the MSM100, wherein the illumination optics is of
low resolution, but the collection optics is of high resolution and controls the
pixel size and resolution.
The novel use of the system to perform aerial imaging mode will be
Preliminary, it should be noted that the light source 230 should
preferably operate at a wavelength comparable to that typically used in a stepper
of interest. For example, a mercury arc lamp may be used for i-line at
265nm for .50-.30 micron design rule technology, while a laser (e.g.,
krypton or Argon Bximers) at the DUV range for .25-.08 design rule technology. This
will improve the resolution in both the conventional and aerial imaging inspection
modes. Additionally, using a wavelength comparable to that used in the photolithography
tool would result in a more "realistic" aerial image.
As shown in Figure 2, an aperture 265 can be selectively
placed so as to selectively alter the effective NA of objective lens 260
(this would be generally referred to as NAill). In general, the objective
lens 260 is of a relatively large numerical aperture (e.g., 0.6) selected
to provide high resolution when aperture 265 is removed, so as to provide
maximum resolution during the conventional inspection mode. However, when aerial
mode is used, it is desired to match the NAill of the inspection system
to that of the exposure tool, e.g., 0.12. Thus, the aperture 265 reduces the effective
NAill from 0.6 to 0.12.
As shown in Fig. 3a, a set of different apertures may be provided
- four in the present example, 28A, 28B, 28C and
28D. The apertures 28A-28C are annular-shaped apertures and the aperture
28D is a quadrupole off-axis aperture enabling the enhancement of depth of
focus (DOF). A selected one of these apertures can be inserted in the optical path
of the laser beam B0. Of course, depending on the specific illumination
desired, other apertures may be used. A modified apperture or a second one may be
used to change the spot shape so that the image shall emmulate the effects of the
photoresist as well.
Additionally, it is desired to change the shape of the light beam
to more closely resemble an exposure tool. To that effect, the illumination aperture
265 may be a diffractive optical element or a proper apodization aperture
affecting also the shape of the incident beam. Preferably, the aperture
265 provides a flat-top beam, i.e., a beam with uniform intensity distribution
over the cross section of the beam. Fig. 3b shows profiles I1
and I2 of the laser beams B6 at the reticle
plane (i.e., the inspection plane) with and without the proper apodization of aperture,
respectively. As known, the primary laser beam has a Gaussian intensity distribution,
profile I1. To convert a Gaussian beam into a flat-top beam having
profile I2, the aperture 265 may be designed like a diffractive
optical beam shaper that changes the propagation phase patterns prior to diffraction
focusing. This beam shaper is one of general classes of diffractive optical elements,
which can be fabricated using computer-generated holograms, photolithography and
ion etching or other methods.
Generally, there is a great variety of beam-shaping techniques aimed
at converting a Gaussian beam into a flat-top beam. Directly truncating the Gaussian
beam with an aperture is a straightforward approach. The Gaussian beam can be attenuated
with a neutral density filter or an electrooptic device having a suitable controlable
transversal transmittance profile. A binary optical beam shaper on interlace diffraction
gratings converts an incident Gaussian beam into an approximately 1-D sinc-square
function beam or a 2-D Bessinc-square fucntion beam in its near field and then generates
a flat-top beam in its far field. Another beam-shaping technique is based on redistributing
the energy of a Gaussian beam with prisms, or aspheric reflective mirrors or aspheric
Also shown in Figure 2 is collection aperture 275, for
adjusting the effective collection numerical aperture NAcol'. Typically,
the aperture 275 is designed to reduce the collection numerical aperture
of a conventional flying spot based inspection system to the stepper associated
value of about 0.15. A condenser lens 270 is optionally used to collect the
light and direct it to the light sensor.
As can be appreciated, when the apertures 265 and 275 of the system
of Figure 2 are inserted into the beam's path, the effective optics of the system
resembles the optics of an exposure tool, except that the system still scans the
reticle using a flying spot Consequently, the optics thus modified can be advantageously
used to obtain an aerial image of the reticle, by scanning the entire reticle in
a serpentine manner. The aerial image can then be compared to a modified database
or evaluated in a die-to-die manner. In operation, the user may wish to inspect
the entire reticle In the conventional mode, then switch to the aerial imaging mode
and inspect the entire reticle in an aerial imaging mode. Alternatively, since the
design of the reticle is known, the user may wish to use the aerial imaging mode
only In areas having dense features, dense OPC's, or phase shift features. Additionally,
the user may wish to use aerial imaging mode to revisit areas indicated as suspect
of having defects during the conventional inspection mode.
In both the conventional and aerial inspection modes the reticle is
scanned using a "flying spot." While such scanning is known in the art, it is summarized
here for completness. As shown in Figure 4, scanner 240 scans the beam in
the fast scan direction to scan a strip 400 of the reticle, while the stage
220 is moved in the slow scan direction to complete a field 420. Using
a serpentine motion, the entire reticle can be inspected.
Returning to Figure 2, an optical beam coherence reducer
235 is depicted as optional equipment. The optical beam coherence reducer
is used in the aerial imaging mode to assist in beam shaping so as to further resemble
an exposure tool. Specifically, the optical beam coherence reducer can be used in
conjunction with the aperture 265 to provide exposure of the reticle that
simulates the exposure provided by an exposure tool.
An optical beam coherence reducer can be made in the form of a rotating
disk. It may be lightly diffusive ground or etched or milled glass as well as diffractive
diffuser with the proper scattering angle and phase shifting pattern. Preferably
the disk rotates so that the surface moves in the contrary direction to the movement
of the laser scanning beam. It is preferably introduced at a location where the
beam is small and not in a place that is imaged on the objective lens.
To change the size of the spot, the system can simply be taken out
of focus or some elements can be moved. That is, the system of Figure 2 is equipped
with a conventional suitable auto-focusing arrangement (not specifically shown),
aimed at maintaining the inspection plane of the reticle In the focal plane of the
objective lens 260. This is generally done by providing motion of the stage
in the Z-axis. Thus, in order to provide effective expension of the beam, the autofocus
can be controlled to set the system out of focus. For example, the stage can be
lowered to a specified distance below the focal point of lens 260.
Also shown in Figure 2 is a dark field detector 215,
which can be operational in either operating mode of the system. When the light
beam hits a transparent area of the mask 210, the light is trasmitted therethrough.
On the other hand, when the light beam hits a reflective chrome area of the mask,
it is reflected back and collected by the objective 260. Under these two
circumstances the dark field detector detects no light and produces no signal. However,
when a particle is present on either the transparent or reflective area, the light
beam is scattered by the particle in various directions and some of that scattered
light is detected by the dark field detector 215. Thus, a very high signal
to noise ratio is generated for the detection of unwanted particles present on the
Reference is made to Fig. 5, illustrating an optical inspection
system 500, constructed and operated according to another embodiment of the
invention. The system 500 is aimed at speeding up the inspection process
by utilizing a multibeam scanning apparatus - two-beam in the present example. The
scanning apparatus comprises a beam splitter and multibeam control mechanism
505 accommodated between the laser source 530 and the deflector element
540. The mechanism 505 splits the primary laser beam B9
into two spatially separated beams B(1)0 and
B(2)0. The beams are separated from each other along
the X-axis, i.e., perpendicular to the scanning direction, and illuminate
two spatially separated spots S1 and S2, respectively,
on the reticle 510. Condenser lens 570 is accommodated in the optical
path of light components B(1)1 and B(2)1,
transmitted through the spots S1 andS2, and
collected by the aperture 575. A detection unit comprises two detectors580A
and 580B for receiving these light components B(1)1
and B(2)1, respectively, and provide appropriate signal
to the processor 515.
The construction of the mechanism 505 does not form part of
the present invention and may be of any known kind. For example, it may include
a beam splitter and a mirror accommodated in the optical path of one of the beams
produced by the passage of the primary beam B0 through the beam
splitter. Generally, the mechanism 505 utilizes a suitable number of beam
splitting means, such as prisms, partially transparent mirrors, etc., and a means
for adjusting the lengths of the optical paths of the beams, e.g., a plane-parallel
plate, so as to impinge onto the deflector element simultaneously. Such multibeam
scanning mechanisms are disclosed, for example, in U.S. Patent Nos. 3,725,574 and
The deflection element 540, e.g., a rotating mirror ao an acousto-optic
deflector (AOD), deflects the beams B(1)0 and B(2)6
and cause them to scan successive spots S1 and successive spots S2,
respectively, on the reticle 510 within spaced-apart parallel identical scan paths
extending along the Y-axis. The scan paths 520A and 520B are
formed by arrays of successively illuminated spots S1 andS2,
respectively (the scanning of which is shown exagerated in Figure 5). At each current
time, a pair of illuminated spots S1 and S2
is inspected, while at each relative location of the reticle relative to the lens
510, a pair of scan paths is inspected.
It should be noted, although not specifically shown, that the processor
unit515 comprises a memory and a programming means for collecting and analyzing
data coming from the detectors. The analysis of the received data includes die-to-die
and/or die-to-database comparison. The use of the dark-field detectors enables the
reticle inspection for pattern and particles related defects simultaneously. The
analysis of the received data includes also the comparison of the data representative
of the dark field scattered light and data representative of the transmitted light.
This transmission-to-reflection comparison is aimed at detecting the so-called "soft
defects", such as particles, damaged antireflection coating, photoresist residuals,
etc. Since the die-to-die and the transmission-to-reflection processing do not occur
at the same time, they may be carried out by the same image processing module.
Those skilled in the art will readily appreciate that various modifications
and changes may be applied to the preferred embodiments of the invention as hereinbefore
described without departing from its scope defined in and by the appended claims.
For example, such operational parameters of the inspection system as light frequency,
numerical aperture and coherence depend on those of the stepper of interest. The
deflection element may be of any known kind. The illumination aperture is also of
any known kind, and is preferably capable of providing a flat-top beam.