111111
`
`1111111111111111111111111111111111111111111111111111111111111111111111111111
`US 20060109455Al
`
`(19) United States
`c12) Patent Application Publication
`Haverlag et al.
`
`(10) Pub. No.: US 2006/0109455 Al
`May 25, 2006
`(43) Pub. Date:
`
`(54) OPTICAL INSPECTION SYSTEM AND
`RADIATION SOURCE FOR USE THEREIN
`
`(75)
`
`Inventors: Marco Haverlag, Eindhoven (NL);
`Daniel Cornelis Schram, Eindhoven
`(NL); Richard Antonius Hendricus
`Engeln, Utrecht (NL); Adrianus
`Henricus Johannes Van Den Brandt,
`Eindhoven (NL)
`
`Correspondence Address:
`PHILIPS INTELLECTUAL PROPERTY &
`STANDARDS
`P.O. BOX 3001
`BRIARCLIFF MANOR, NY 10510 (US)
`
`(73) Assignee: Koninklijke Philips Electronics N.V.,
`Eindhoven (NL)
`
`(21) Appl. No.:
`
`10/536,231
`
`(22) PCT Filed:
`
`Nov. 26, 2003
`
`(86) PCT No.:
`
`PCT /IB03/05857
`
`(30)
`
`Foreign Application Priority Data
`
`Nov. 28, 2002
`
`(EP) ........................................ 02080004.1
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`(2006.01)
`GOJN 21188
`(52) U.S. Cl. .......................................................... 356/237.2
`
`(57)
`
`ABSTRACT
`
`Disclosed is a system for inspecting an object. Such system
`comprises; a) an irradiation system for irradiating the object
`to be inspected, said irradiation system comprising a radia(cid:173)
`tion source, b) an objective, imaging the irradiated object
`onto an image sensor, and c) an image sensor for transform(cid:173)
`ing the radiation coming from the object to be inspected into
`a detectable signal. The radiation source comprises; A) at
`least one cathode, B) at least one anode, C) one or more
`plates, positioned in between said cathode(s) and said
`anode(s), and being electrically substantially insulated,
`wherein each plate comprises at least one hole, aligned in
`such a way that a continuous path is created between cathode
`and anode over which a discharge can extend.
`
`9
`
`17
`
`Energetiq Ex. 2071, page 1 - IPR2015-01362
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`

`
`Patent Application Publication May 25, 2006 Sheet 1 of 2
`
`US 2006/0109455 A1
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`Energetiq Ex. 2071, page 2 - IPR2015-01362
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`

`
`Patent Application Publication May 25, 2006 Sheet 2 of 2
`
`US 2006/0109455 Al
`
`._1 _..,.......___.1--39
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`Energetiq Ex. 2071, page 3 - IPR2015-01362
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`

`
`US 2006/0109455 AI
`
`May 25,2006
`
`1
`
`OPTICAL INSPECTION SYSTEM AND
`RADIATION SOURCE FOR USE THEREIN
`
`[0001] The present invention relates to a system for
`inspecting an object, comprising:
`
`[0002] an irradiation system for irradiating the object to be
`inspected, said irradiation system comprising a radiation
`source,
`
`[0003] an objective, imaging the irradiated object onto an
`image sensor, and
`
`[0004] an image sensor for transforming the radiation
`coming from the object to be inspected into a detectable
`signal.
`
`In the above inspection system, the radiation that
`[0005]
`comes from the object to be inspected generally contains
`information on patterns that are present in or on the object,
`thicknesses of layers on the object and/or compositions of
`materials on the object.
`
`[0006] Optical inspection systems are widely used for
`inspecting objects. For example, in the semiconductor indus(cid:173)
`try use is made of-automatic-wafer inspection systems.
`Such wafer inspection systems are used for the inspection of
`the quality of wafer processing in order to detect processing
`defects, layer thicknesses and/or contamination on the
`wafers.
`
`[0007] During the processing of a wafer an array of
`patterns is placed on the wafer and each pattern has to be
`placed with submicron precision, as line widths and elemen(cid:173)
`tal areas are very small. Successive layers are to be built up
`for each pattern on a wafer and these have to be carefully
`checked before any further processing can be undertaken.
`Optical inspection is used to determine whether any defects
`have been introduced, such as for example misalignment,
`electrical shortcuts, and impurities.
`
`[0008] Currently, in wafer inspection systems, broadband
`radiation from high power Hg/Xe arc lamps is used. These
`Hg/Xe arc lamps yield radiation in the range of 200-700 nm,
`of which predominantly the shorter wavelengths are used.
`
`[0009] However, the semiconductor industry constantly
`reduces the feature sizes on the wafers. Line widths of 80-90
`nm are applied at present, while not long ago the minimum
`line width was 200 nm. This line width reduction requires
`the use of a radiation source with smaller wavelengths in the
`wafer inspection system in order to be able to see the smaller
`details.
`
`[0010] Next to that, in order to operate the wafer inspec(cid:173)
`tion systems effectively, there is a minimum required in the
`radiation flux on the inspected products to reach the required
`inspection speed (in wafers/hour).
`
`[0011] With the current Hg/Xe arc sources the options
`both to provide for shorter wavelengths and to increase the
`radiant flux on the inspected products are limited due to the
`absence of emitted radiation at shorter wavelengths as well
`as the limited radiance at larger wavelengths.
`
`[0012] The present invention aims to provide for a solu(cid:173)
`tion to the above problem. To that end, the present invention
`provides for an inspection system according to the preamble
`that is characterized in that the radiation source comprises:
`
`[0013] at least one cathode
`
`[0014] at least one anode
`
`[0015] one or more plates, positioned in between said
`cathode(s) and said anode(s), and being electrically substan(cid:173)
`tially insulated, wherein each plate comprises at least one
`hole, aligned in such a way that a continuous path is created
`between cathode and anode over which a discharge can
`extend.
`
`In the above system, the plates are in particular
`[0016]
`placed in a cascade and are electrically substantially insu(cid:173)
`lated from each other, from the cathode( s) and from the
`anode(s).
`
`[0017] The arrangement of the radiation source is also
`referred to as a cascade arc radiation source. A cascade arc
`source is, for example, disclosed in U.S. Pat. No. 4,871,580,
`which is incorporated herein by reference. A cascade arc
`comprises three major sections; a cathode section, an anode
`section and a plate section in-between. The plate section,
`which typically comprises several plates with holes stacked
`into a cascade, gives the arc its name. Upon operation, an
`electrical current is flowing from the anode to the cathode,
`through the holes in the cascade plates, creating plasma that
`generates light.
`
`[0018] A cascade arc source provides for a radiance that is
`much higher than the radiance of the common Hg/Xe arc
`sources. The cascade arc source emits its flux in a very small
`geometrical extent and has a radiance of approximately 0.1
`W/nm/mm2/sr There are two main benefits of this source
`with respect to the currently used high power Hg/Xe arc
`lamps. First of all, the source emits light at shorter wave(cid:173)
`lengths, allowing a higher spatial resolution. The cascade arc
`source emits radiation in a wavelength range from below
`125 nm to the infrared. A range of 120-400 nm, or when
`preferred, in view of absorption in quartz below 190 nm,
`190-400, can easily be reached. Moreover, the source has a
`small geometrical extent enabling much larger magnifica(cid:173)
`tions of the object to be inspected, even at very high speed.
`
`[0019] Preferably, the inspection system according to the
`invention provides for a wavelength region that is limited to
`any band or set of bands of wavelengths, comprising at least
`radiation at wavelengths of at least 190 nm.
`
`[0020] Radiation at these shorter wavelengths from 190
`nm is for example very favorable when the inspection
`system is used for inspecting semiconductor devices.
`
`[0021] Advantageously, the radiation source produces
`radiation with radiance larger than 10 mW/nm/mm2/stera(cid:173)
`dian.
`
`[0022] Although in principle all kind of objects could be
`inspected by the system according to the present invention,
`the system is in particular suitable for inspecting bare,
`partially or fully processed semiconductor wafers or reticles
`or masks used in a lithographic process to produce a
`patterned layer on a semiconductor wafer.
`
`[0023] Preferably, the irradiation system comprises optical
`means for homogenizing the spatial distribution of the
`irradiation in the image plane on the object. In particular, the
`optical means comprise a homogenizer.
`
`[0024] Such homogenizer conditions the light coming
`from the radiation source and homogenizes the spatial
`distribution thereof.
`
`[0025] The present invention also relates to a radiation
`source as disclosed in the above for use in an optical
`inspection system.
`
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`

`
`US 2006/0109455 AI
`
`May 25,2006
`
`2
`
`[0026] The present invention will be illustrated with ref(cid:173)
`erence to the drawing, in which:
`
`[0027] FIG. 1 schematically shows a cascade arc source;
`and
`
`[0028] FIG. 2 schematically shows an optical design for
`wafer inspection according to the invention.
`
`[0029] The figures are purely schematic and not drawn to
`scale. Similar elements will be referred to with the same
`reference numerals as far as possible.
`
`[0030] FIG.l shows a cascade arc radiation source 1. Said
`cascade arc comprises three major sections; a cathode sec(cid:173)
`tion, an anode section and a plate section in-between. The
`plate section, which comprises several plates stacked into a
`cascade, gives the arc its name.
`
`[0031] The exemplary construction of a cascade arc as
`shown in FIG. 1, comprises a central channel 3 having a
`length varying from 20-200 mm and a diameter varying
`from 0.5-10 mm.
`
`[0032] Reference numeral 6 indicates a cathode tip. In
`FIG. 1 only one cathode tip is shown. In practice the amount
`of cathodes is variable, but preferably at least three cathode
`tips will be present. Advantageously, the cathode tips com(cid:173)
`prise an alloy of thorium in tungsten. The cathode tips are
`preferably arranged rotatively aronnd the central channel 3
`and are mounted in hollow holders 8 through which cooling
`water is fed via duct 9. The holders 8 are at least partially
`enclosed in an electrically insulating sleeve 10, for example
`made from quartz, and are held in position by a screw 11
`which accommodates a rubber ring-not shown in FIG.
`l-and which clamps the holder 8 vacuum tight. The duct 9
`is clamped tight in the holder 8 by a screw 12.
`
`[0033] The cathode tips 6 can easily be replaced by
`removing the holder 8 from the arc assembly, replacing the
`cathode tip 6 and putting the holder with the new tip back in
`the assembly.
`
`[0034] Reference numeral 4 indicates an inlet through
`which a flushing gas can be fed. As examples of such
`flushing gas noble gases like argon and xenon can be
`mentioned. Reference numeral 5 indicates a nozzle-like
`anode that is located at the end of channel 3 opposite the
`cathodes 6. In the embodiment shown, the anode 5 com(cid:173)
`prises an easily removable conical insert that is placed into
`a conical hole in a water-cooled plate 15. Cooling water is
`fed to this cooling-plate 15 via inlet 16 and discharged
`therefrom via the outlet 17.
`
`[0035] A stack of cascade plates 14 are affixed to the anode
`plate 15 by a bolt 18 and a nut 19. Electrical insulation is
`achieved by the presence of sleeve 20, cap 21, and rings 22
`and 23. The cascade plates 14 are for example made of
`copper. Due to the high temperatures in the cascade arc-up
`to over 16,500 K-the plates must be cooled. The cooling
`liquid channels, which are not shown in FIG. 1, are close to
`the central channel 3, resulting in good heat dissipation. The
`cascade plates 14 are separated from one another and
`electrically insulated by means of a sealing system of "0"
`rings 24, spacers 25 (e.g. PVC spacers), and central rings 26
`made of boron-nitride. The seals ensure that the arc can be
`maintained at pressures between 0.05 and 20 bar. The central
`rings are white colored and reflect the light radiating from
`the plasma. The object of the central rings is to act as
`protection for the "0" rings against melting under the
`influence of plasma light absorption.
`
`[0036] During operation of the cascade arc a direct-current
`electricity of between 20-200 A can flow from the nozzle
`like anode 5 to the cathode tips 6. Operation of the cascade
`arc using pulsed electrical currents is also possible.
`
`[0037] The arc can be ignited by first lowering the gas
`pressure to approximately 10 mbar and applying a voltage
`difference of approximately 1000 V between the anode and
`the cathode (voltage depending on, amongst others, elec(cid:173)
`trode-distance). Once the arc is ignited, the pressure of the
`gas is increased to operating pressure, i.e. between 0.05-20
`bars. Other ignition procedures are also conceivable.
`
`[0038] FIG. 2 represents schematically an example of an
`optical design of a wafer inspection system according to the
`invention. It will be clear that within the scope of the
`invention many other designs are possible. Reference
`numeral 32 refers to an illumination system. In the present
`example, said system comprises a cascade arc 31 that is used
`as a light source. Furthermore, the illumination system 32
`comprises means 34 for spatially homogenizing the beam
`intensity at the exit 33 of the illumination system 32.
`
`[0039] The light beam that leaves the illumination system
`through exit 33 passes collimator lens system 35. Numeral
`38 refers to the objective of the inspection system. Reference
`numeral 39 indicates an image sensor. The objective both
`images the light beam onto the wafer 37 and collects the
`reflected light from the wafer. With a beam splitter 36 the
`light is directed out of the illumination path and together
`with the objective an image of the wafer is made.
`
`[0040] Although the present invention is illustrated by
`means of the above examples, it is not intended that the
`invention is limited to these examples. On the contrary,
`many variations are possible within the scope of the present
`invention.
`
`1. System for inspecting an object, comprising:
`
`an irradiation system for irradiating the object to be
`inspected, said irradiation system comprising a radia(cid:173)
`tion source,
`
`an objective, imaging the irradiated object onto an image
`sensor, and
`
`an image sensor for transforming the radiation coming
`from the object to be inspected into a detectable signal,
`characterized in that the radiation source comprises:
`
`at least one cathode
`
`at least one anode
`
`one or more plates, positioned in between said cathode(s)
`and said anode(s), and being electrically substantially
`insulated, wherein each plate comprises at least one
`hole, aligned in such a way that a continuous path is
`created between cathode and anode over which a
`discharge can extend.
`2. Inspection system according to claim 1, characterized
`in that the wavelength region is limited to any band or set of
`bands of wavelengths, comprising at least radiation at wave(cid:173)
`lengths of at least 190 nm.
`3. Inspection system according to claim 1, characterized
`in that the radiation source produces radiation with a radi(cid:173)
`ance larger than 10 mW/nm/mm2/steradian.
`4. Inspection system according to claim 1, characterized
`in that the irradiated object is a bare, partially or fully
`processed semiconductor wafer.
`
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`

`
`US 2006/0109455 AI
`
`May 25,2006
`
`3
`
`5. Inspection system according to claim 1, characterized
`in that the irradiated object is a reticle or mask used in a
`lithographic process to produce a patterned layer on a
`semiconductor wafer.
`6. Inspection system according to claim 1, where the
`irradiation system comprises optical means for homogeniz(cid:173)
`ing the spatial distribution of the irradiation.
`
`7. Inspection system according to claim 6, characterized
`in that the optical means comprise a homogenizer.
`8. Radiation source as disclosed in claim 1 for use in an
`optical inspection system.
`
`* * * * *
`
`Energetiq Ex. 2071, page 6 - IPR2015-01362