(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2008/0055712 A1
`Noelscher et al.
`(43) Pub. Date:
`Mar. 6, 2008
`
`US 200800557l2A1
`
`(54) FILTER SYSTEM FOR LIGHT SOURCE
`
`Publication Classification
`
`(76)
`
`Inventors:
`
`Christoph Noelscher, Nuemberg
`(DE); Sven Trogisch, Dresden
`(DE)
`
`Correspondence Address:
`SLATER & MATSIL LLP
`
`17950 PRESTON ROAD’ SUITE 1000
`DALLAS: TX 75252
`
`(21) Appl. No‘:
`
`11/513,502
`
`(22)
`
`Filed:
`
`Aug. 31, 2006
`
`(51)
`
`Int.(3L
`(200601)
`G02F 1/33
`(52) U.S. Cl.
`..................................................... .. 359/308
`(57)
`ABSTRACT
`
`The invention is concerned with a filter system for a light
`source in a lithography process for the production of semi-
`conductor devices with a flowing absorber gas for at least
`one wavelength (A) in the range between 20 to 250 nm the
`flowing absorber gas intersecting the light path emitted by
`the light source. Furthermore, the invention is concerned
`with a lithography apparatus for processing semiconductor
`substrates, the use of a filter system, a method for filtering
`light and a semiconductor device manufactured by the
`method.
`
`ASML 1240
`ASML 1240
`
`

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`Patent Application Publication Mar. 6, 2008 Sheet 1 of 7
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`US 2008/0055712 A1
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`Patent Application Publication Mar. 6, 2008 Sheet 2 of 7
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`US 2008/0055712 A1
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`Patent Application Publication Mar. 6, 2008 Sheet 3 of 7
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`US 2008/0055712 A1
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`

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`Patent Application Publication Mar. 6, 2008 Sheet 4 of 7
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`US 2008/0055712 A1
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`FIG 3
`
`23%
`
`

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`Patent Application Publication Mar. 6, 2008 Sheet 5 of 7
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`US 2008/0055712 A1
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`

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`Patent Application Publication Mar. 6, 2008 Sheet 6 of 7
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`US 2008/0055712 A1
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`FIG 5
`
`23
`
`EUV, DUV
`
`

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`Patent Application Publication Mar. 6, 2008 Sheet 7 of 7
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`US 2008/0055712 A1
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`300
`
`2w
`
`200
`
`
`
`100150 Lambda[nm]
`
`50
`
`Z
`
`0-mmmmmmmmw
`
`C\|
`
`5
`1—
`
`0
`V—
`
`5
`
`LUU 1;‘gL pure epqum 12 SOHBJ uouoas ssom uondiosqv
`
`FIG6 2m
`
`

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`US 2008/0055712 A1
`
`Mar. 6, 2008
`
`FILTER SYSTEM FOR LIGHT SOURCE
`
`TECHNICAL FIELD
`
`[0001] This invention relates generally to a filter system
`for a light source in a lithography process in the production
`of semiconductor devices. Furthermore, the invention also
`relates to a filtering method and semiconductor devices
`manufactured with the filtering method.
`
`BACKGROUND
`
`[0002] The current processes in the field of lithography
`requires shorter and shorter wavelengths because of the
`increasing trend to manufacture smaller and smaller struc-
`tures for semiconductor devices such as DRAM chips.
`Usually, a resist, i.e. a chemical substance sensitive to light,
`is illuminated by a light source via a mask. A pattern on the
`mask (either a reflective mask or a transmission mask) is
`transferred to the light sensitive resist which in turn is used
`in further processing steps to generate the structures on a
`substrate.
`
`For the illumination light with wavelengths of 248
`[0003]
`nm, 193 nm or 157 nm are used. Those wavelengths are
`generally termed as deep UV light.
`[0004] The latest step in the direction of smaller wave-
`lengths is the usage of extreme ultra violet (EUV) light with
`wavelengths between 1
`to 50 nm,
`in particular with a
`wavelength of 13.4 nm. This EUV light is usually produced
`by the generation of plasmas. It is known e.g. to generate
`EUV light with the wavelength of 13.4 nm with a Xenon gas
`ignited by a pulsed Nd:YAG Laser to produce a plasma.
`[0005] But it is also known that such EUV generating
`plasmas emit light at other wavelengths, in particular in the
`deep ultra violet range (100 to 300 nm). Resists used in
`lithography are generally not only sensitive for light of one
`particular wavelength but for light in a range of wave-
`lengths. Therefore, a resist for EUV lithography exhibits
`also sensitivity for light at DUV wavelengths reducing the
`quality of the lithography result.
`
`SUMMARY OF THE INVENTION
`
`[0006] The invention is concerned with a filter system and
`a method for filtering light from a light source in a lithog-
`raphy process in the production of semiconductor devices
`with a flowing absorber gas for at least one wavelength A in
`the range between 30 to 250 nm, the flowing absorber gas
`intersecting the light path emitted by the light source.
`Furthermore, the invention is concerned with semiconductor
`devices, manufactured with said method.
`[0007]
`It is the purpose of the absorber gas to filter out
`some of the light at higher wavelengths.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0008] Embodiments and advantages of the invention
`become apparent upon reading of the detailed description of
`the invention, and the appended claims provided below, and
`upon reference to the drawings.
`[0009]
`FIG. 1 schematically shows a first embodiment of
`a EUV lithography system;
`[0010]
`FIG. 1A schematically shows a second embodi-
`ment of a EUV lithography system;
`[0011]
`FIG. 2 schematically shows a third embodiment of
`a filter system using a free flowing absorber gas stream;
`
`FIG. 3 schematically shows a fourth embodiment
`[0012]
`being a variation of the first embodiment;
`[0013]
`FIG. 4 schematically shows a fifth embodiment
`being a variation of the first embodiment;
`[0014]
`FIG. 5 schematically shows a sixth embodiment of
`a filter system using a filter chamber;
`[0015]
`FIG. 6 showing the absorption cross section of
`different absorber gases.
`
`DETAILED DESCRIPTION OF ILLUSTRATIVE
`EMBODIMENTS
`
`[0016] The following embodiments of the invention are
`described in connection with an extreme ultra violet (EUV)
`system used in a lithographic process in the manufacturing
`of semiconductor devices. Examples for semiconductor
`devices which can be manufactured by such systems are
`DRAM memory chips, NROM memory chips and micro-
`processors.
`[0017] As depicted in FIG. 1 a light source 1 emits EUV
`radiation along a light path 2 primarily at 13.4 nm from
`Xenon plasma generated within the source 1.
`[0018]
`It is important to note that the invention is not
`limited to such light sources 1 because light sources 1 for
`other wavelengths can also emit considerable energy at EUV
`wavelengths so that
`the filter system according to the
`invention can be used for other systems as well.
`[0019]
`From the light source 1 a light cone is emitted
`along the light path 2 to a wafer 10. It is the aim of the
`lithography system to generate structures on the wafer 10 in
`a principally known way. The structures are imprinted on a
`reflection mask 3 which guides the light from the light
`source 1 onto the wafer 3. The light projected onto the wafer
`10 interacts with an optical sensitive layer (resist).
`[0020]
`In FIG. 1 a first mirror system 4A is shown which
`guides the light along the light path 2 from the source 1 to
`the mask 3.
`
`[0021] After the mask 3 a second mirror system 4B is
`positioned guiding the light to the wafer 10. Typically the
`mirror systems 4A, 4B comprise a plurality of mirrors.
`[0022] Behind the light source 1, but before the first mirror
`system a filter system 100 is positioned. In the following
`some exemplary embodiments of such a filter system 100
`are described.
`
`In FIG. 1A a second embodiment of an EUV
`[0023]
`lithography system is schematically shown. In this the filter
`system 100 is positioned within the first mirror system 4A,
`here at an intermediate focus point.
`[0024]
`In connection with FIG. 1 and FIG. 1A the filter
`system 100 is shown as a distinct unit, positioned within the
`EUV lithography system. Alternatively, the flowing absorber
`gas 20 can be flowing throughout the internal space of the
`EUV lithography system, so that the light path 2 takes a long
`way through the absorber gas 20. To protect mirrors in the
`mirror systems 4A, 4B, the region around the mirrors could
`be kept free from the absorber gas 20 by selectively pumping
`away absorber gas 20 around mirrors or by selectively
`removing the absorber gas 20 with an inert gas flow.
`[0025]
`In FIG. 2 the filter system 100 as a part of the
`lithography system is shown schematically. Between the
`light source 1 and the rest of the system (e.g. mirrors, mask,
`wafer etc.) a free flowing absorber gas 20 is introduced in the
`light path 2. In this first embodiment the absorber gas 20 is
`introduced between the light source 1 and a not depicted first
`mirror of the lithography system. Alternatively, it is possible
`
`

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`US 2008/0055712 A1
`
`Mar. 6, 2008
`
`to introduce the absorber gas 20 stream exclusively or
`additionally at other locations in the light path 2.
`[0026] As mentioned in context with FIG. 1, the light
`source 1 emits EUV light but also other wavelengths (e.g.
`DUV), especially in the range between 20 to 250 nm.
`Therefore, the spectrum of the light source 1 has a broader
`spectrum as desired.
`[0027]
`It is the purpose of the absorber gas 20 to filter out
`the wavelengths in the range between 20 and 250 nm so that
`predominantly EUV light is emitted. The absorber gas 20 is
`chosen to have an effective absorption cross section for the
`undesirable wavelengths.
`[0028]
`In this context absorption cross section is defined
`by the Lambert-Beer law describing the attenuation of light
`by a homogeneous absorbing system:
`1:10 exp (0 (in)
`
`where IO and I are the incident and transmitted light inten-
`sities, d is the absorption path length (in cm), n is the
`concentration of the absorber (in molecule/cm3), and 0 is the
`absorption cross section (in cm2 molecule”).
`[0029]
`In the described embodiment the absorber gas 20 is
`phosphine (PH3). Alternatively, benzene (C6H6), carbon
`disulfide (CS2), hydrogen, ethylene (CH2CH2), helium,
`neon, argon, krypton or xenon could be used.
`[0030]
`Furthermore, it is within the scope of the invention
`to use mixtures of two or more different absorber gases 20.
`The composition of the gas mixture can be chosen to
`optimize the absorption properties. This and other properties
`of the absorber gas 20 will be discussed in connection with
`FIG. 6.
`
`In this first embodiment the absorber gas 20 is a
`[0031]
`free flowing stream intersecting the light path 2. In this
`embodiment the flow is essentially perpendicular to the light
`path 2. If it is necessary, the flow direction can be at an angle
`to the light path 2.
`[0032] The internal space of the lithography system is
`usually evacuated, i.e. operating at a vacuum (e.g. a range
`between 10 mbar to 10”‘) mbar). Therefore, the absorber gas
`20 flow expands into the vacuum and has to be removed
`sufiiciently before an undesired contamination of other
`equipment takes place. A number of measures can be taken
`to that effect. A person skilled in the art understands that
`those measures shown in the figures can be employed
`individually or in any possible combination.
`[0033]
`First of all the gas flow of the absorber gas 20 is
`directed by a nozzle 21 to give the gas flow a directed
`profile. For the purpose of this embodiment it is deemed
`advantageous that the drift velocity of the absorber gas 20,
`i.e. the velocity from the inlet of the absorber gas 20 to the
`outlet,
`is at least as high as the thermal velocity, i.e. the
`velocity of a molecule in one direction due to thermal
`fluctuation.
`In the limit,
`the thermal fluctuations would
`widen the absorber gas stream to a cone with a cone angle
`of 90°.
`
`[0034]
`to be
`
`The thermal velocity in one direction can assumed
`
`RT
`Vthermal = —
`
`M is the molecular weight, R is the gas constant, and T is the
`temperature.
`
`[0035] At room temperature phosphine would have a
`thermal velocity of approximately 270 n1/s. This would be
`the minimum velocity of the drift velocity for this particular
`absorber gas and at this temperature. Increasing the drift
`velocity, by increasing e.g. the pressure difference, would
`result in a narrower cone angle of the absorber gas stream
`20. A doubling of the drift velocity to 540 m/s would reduce
`the cone angle from 90° to 53°. The cone angle can be
`calculated from
`
`
`
`Vthermal Jconeangle = 2 >1: arctan[ Vdnfi
`
`[0036] The relationship for the thermal velocity shows that
`a cooling of the absorber gas 20 results in a lowering of the
`velocity. This would result in a smaller cone angle.
`[0037]
`In the embodiment depicted in FIG. 2 the absorber
`gas 20 is recycled by recycling means 23. Therefore, the free
`flowing absorber gas 20 is collected by collector means 22
`(e.g. stainless steel plates).
`[0038] The composition of the absorber gas 20 will be
`altered by the irradiation in the light path 2. Therefore, a
`filter unit 24 is used in the recycle 23 to filter out undesired
`parts of the absorber gas. The filter unit 24 can use e.g.
`membranes, distillation, absorption, adsorption, condensa-
`tion or a combination of those unit operations. Fresh
`absorber gas 24A might be introduced to make up for losses
`24B.
`
`Since absorber gas is heated by the light emitted
`[0039]
`from the light source 1, a cooler 25 is employed to bring the
`recycled absorber gas 20 into the appropriate temperature
`range. The cooler 25 can be integrated into the recycle 23,
`i.e. the tubing itself would act as cooler 25. The temperature
`in this example is room temperature. Alternatively,
`the
`temperature can be lower than room temperature, limiting
`the thermal velocity. Alternatively, the temperatures can be
`higher. The absorber gas 20 can have temperatures between
`10 K and 600 K.
`
`[0040] The whole recycle of the absorber gas 20 is main-
`tained by a pump system 26. It might be advantageous to
`operate the pump system 26 in a pulsed mode to synchronize
`the introduction of the absorber gas 20 with a pulsed light
`source 1. Typical pulse rates are in the range of 4 to 10 kHz.
`[0041] The advantageous effect of the embodiment is a
`much higher ratio of EUV light reaching the resist on the
`wafer 10 than without the absorber gas.
`[0042]
`In a typical lithograph system with an EUV plasma
`light source 1, only 50% of the EUV intensity reaches the
`wafer. This
`situation can be improved by employing
`embodiments of the invention.
`
`If the input absorption of EUV light is set to 10%,
`[0043]
`the absorption of EUV (aEUV) can be calculated from
`l0%:IEUV_0-(l—exp(—aEUV))
`
`[0044] With the EUV intensity without filter IEUV_O:l:
`aEUV:0,l054
`[0045]
`Setting a ratio of the absorption coefiicient ratio
`between DUV and EUV to 20, the absorption coefficient for
`DUV can be derived from
`
`

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`US 2008/0055712 A1
`
`Mar. 6, 2008
`
`0
`
`" aEUV 2 aDUV = 2,1072
`aDUV
`
`[0046] The absorption of DUV is then
`1—IDUV0-(exp(—aDUV)):87.8%
`
`[0047] Consequently, the output DUV light compared to
`the input is
`—exp(—aDUV):12.2%
`
`[0048] The ratio of DUV output to EUV output is then
`6.8%.
`
`[0049] An absorber gas with an absorption ratio aDUV/
`aEUV of 20 and an input absorption for EUV light of 10%
`would absorb 88% of the DUV light. This would be a ratio
`of DUV/EUV output of 6.8%, down from 50%.
`[0050]
`In FIG. 3 a modification of the first embodiment is
`shown so that reference is made to the description of FIG.
`2. Unlike in the first embodiment, the absorber gas 20 is
`ionized so that it can be controlled by an electric field 28
`(only schematically depicted in FIG. 3). The absorber gas
`flow and the electric field are oriented in the same direction.
`
`[0051] The absorber gas 20 is ionized by an ionizer 29
`which couples e.g. microwave or RF radiation into the
`absorber gas.
`is altered by adding an
`[0052] The first embodiment
`electrical field means 27 to the collector means 22. By
`applying the appropriate polarity to the field means 27, the
`ionized absorber gas 20 stream is directed towards the
`collector means 22. This helps in reducing possible internal
`contamination of the system.
`[0053]
`In FIG. 4 a further embodiment is shown. This
`embodiment directs the absorber gas 20 into the EUV
`lithographic system by using various apertures A1, A2, A2‘,
`A1‘ connecting spaces having different pressures po, pl, p2,
`p l., po. due to differential pumping.
`[0054]
`FIG. 4 shows a variation of the setup depicted in
`FIG. 2, so that reference can be made to the relevant
`description. Unlike the embodiment in FIG. 2, the light path
`2 is directed through a tube like housing 30 which is
`subdivided by apertures A1, A2, A2‘, A1‘ into five compart-
`ments. The openings in the apertures A1, A2, A2‘, A1‘ are
`just large enough to let the light beam through. They are
`especially positioned at places where the light beam is
`narrow.
`
`[0055] The absorber gas flow 20 flows through the middle
`compartment and is intersected by the light path 2. In
`addition to the first pump 26, second, third and fourth pumps
`26A, 26B, 26C are positioned downstream to the absorber
`gas flow 20, each of the three pumps 26A, 26B, 26C being
`connected to three of the compartments in the housing 30.
`The second pump 26A is connected to the same compart-
`ment the absorber gas is discharged by the first pump 26.
`The highest pressure P2 is present in this compartment. This
`pressure P2 is chosen to be sufiiciently high to allow for
`absorption of the light.
`[0056]
`To the left and the right of this compartment, the
`pressures pl p l. are lower than pressure p2 due to pressure
`loss at the apertures A2, A2‘ and the pumping effect of the
`third and fourth pumps 26B, 26C.
`
`[0057] The leftmost and rightmost compartments in the
`housing contain basically vacuum. Therefore, the pressure
`differentials
`are
`(from left
`to
`right
`in FIG.
`4):
`Po<P1<P2>P1'>Po~
`
`[0058] The absorber gas 20 leaving the second, third and
`fourth pumps 26A, 26B, 26C is collected together, recycled
`and treated as mentioned previously.
`[0059] The advantage of this system is that the absorber
`gas 20 is guided by the housing 30 lowering the risk of
`contamination of mirrors but it is not necessary to have a
`closed chamber (see FIG. 5) for the absorber gas 20. The
`differential pumping, i.e. the creation of pressure differences
`between vacuum and the absorber gas 20 containing com-
`partments confines the absorber gas 20 to a relatively narrow
`area. By adjusting the pressure levels in the compartments
`by a computer control 50, the absorption properties of the
`system can be adjusted. Furthermore, a variant of this
`embodiment might contain less or more compartments with
`different pressure levels.
`In the simplest case just one
`compartment is used.
`[0060]
`Preferably, there is no mirror within the tubular
`housing 30 to prevent the contamination of the mirror by
`absorber gas 20.
`[0061]
`FIG. 5 shows another embodiment in which the
`absorber gas 20 flow is not a free flowing gas stream but a
`stream passing through an absorber chamber 30. The
`absorber gas chamber 30 has at least one wall made of
`beryllium in those parts which are subjected to the light of
`the light source 1. Beryllium is essentially transparent to the
`light of the light sources 1.
`[0062] The introduction of the absorber chamber 30
`reduces the risk of contaminating the interior of the lithog-
`raphy system. The recycling, cooling, filtering and pumping
`is facilitated as in the previously mentioned embodiments so
`that reference is made to the relevant description. Altema-
`tively, the guiding of the absorber gas 20 stream can be
`enhanced by an electric guiding means 27 as is shown in
`context with FIG. 3.
`
`In FIG. 5 several substances which can be used as
`[0063]
`absorber gases 20 are characterized. FIG. 5 depicts the
`absorber cross section ratio at a wavelength 2» relative to the
`absorber cross section at 13.4 nm (EUV light). The absorber
`cross section has been defined above.
`
`Phosphine (PH3) shows a significant absorption
`[0064]
`over the range from 50 to 200 nm so that this particular
`absorber gas 20 is effective in an embodiment for a filter
`system. Phosphine can be used alone or in a mixture with
`other absorber gases.
`[0065] Another effective absorber gas 20 could be benzene
`(C6H6) which is less effective at higher wavelengths, but
`more effective at lower wavelengths.
`[0066] An example for a mixture of absorber gases 20 is
`a mixture of carbon disulfide (CS2) and ethylene (CHZCH2).
`Ethylene is more effective at lower wavelengths, carbon
`disulfide at higher wavelengths.
`[0067]
`In another embodiment different absorber gases 20
`(such as the ones named above e.g.) are used not in a mixture
`but
`in separate chambers in the light path 2. Thereby
`possible interactions of the gases are prevented.
`[0068] The absorber gas 20 (or the mixture of gases used
`as an absorber gas 20) is not limited to the species given
`above, i.e. many other gases are suitable to be used in a filter
`
`

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`US 2008/0055712 A1
`
`Mar. 6, 2008
`
`system according to the invention. It is advantageous that the
`absorber gas 20 (or the mixture) fulfills at least the following
`condition:
`
`[0083] Another advantageous criterion for the absorber
`gas 20 can be based on the SEE:
`
`1]:
`
`Absorption cross section for the at
`least one wave length interval A 1 5 nm
`Absorption cross section for a
`reference wave length of 13, 4 nm
`
`[0069] The numerator uses the definition for an absorption
`cross section (e.g.
`the one given above) for a relatively
`narrow wavelength band around a particular wavelength.
`[0070] The denominator provides some reference by com-
`paring the value of the numerator against the wave length of
`13.4 nm,
`i.e. the EUV wavelength. If the ratio 1125 the
`particular wavelength (including the small band) is five
`times more efficiently absorbed than the reference wave
`length. Other advantageous embodiments for the absorber
`gas have 11210 or @220.
`[0071] Another criterion for an absorber gas 20, which can
`be used independently or cumulative with the above defi-
`nition uses a wavelength dependent spectral exposure effi-
`ciencies (SEE).
`[0072] With an absorber gas, the SEE is defined as:
`SEE(}\.):IO\.)'(1-/1(}\.))'S(}\.)
`
`[0073] Without an absorber gas, the SEE is defined as:
`SEEo(7~):1(7~)'(1-Ao(?»))'5(?»)
`
`[0074] With
`[0075]
`l(A):spectral intensity of the light source 1 (incl.
`the reflectivity of the mirrors 4A, 4B)
`[0076] A(A):spectral intensity attenuation along the light
`path 2 with the absorber gas 20
`[0077] AO(A):spectral intensity attenuation along the light
`path without the absorber gas
`[0078]
`S(A):sensitivity of the photoresist on the wafer 10
`[0079] Based in this, a filter efficiency FE can be defined:
`
`250nm
`SEE0(A)dA
`f
`FE = Mn}
`Zfffm SEE(A)dA
`
`SEE0 (A)
`SEE(A) > 5
`
`for at least one wavelength in the range between 20 and 250
`nm.
`What is claimed is:
`
`1. Filter system for a light source in a lithography process
`for the production of semiconductor devices with a flowing
`absorber gas for at least one wavelength (A) in the range
`between 20 to 250 nm, the flowing absorber gas intersecting
`the light path emitted by the light source.
`2. Filter system according to claim 1, wherein the
`absorber gas has the property
`
`Absorption cross section for the at
`= 2
`least one wave length interval A 1 5 nm 5
`Absorption cross section for a
`reference wave length of 13, 4 nm
`
`7]
`
`3. Filter system according to claim 1, wherein the
`absorber gas has the property
`
`Absorption cross section for the at
`least one wave length interval A 1 5 nm
`1] : . .
`
`Absorption cross section for a
`reference wave length of 13, 4 nm
`
`2 10
`
`4. Filter system according to claim 1, wherein the
`absorber gas has the property
`
`Absorption cross section for the at
`least one wave length interval A 1 5 nm
`1] : . .
`
`Absorption cross section for a
`reference wave length of 13, 4 nm
`
`2 20
`
`5. Filter system according to claim 1, wherein the
`absorber gas having the property:
`
`[0080] Unlike in the above given ratio 11, the integral ratio
`of SEEs, i.e. the filter efficiency FE factors in the sensitivity
`of the resist.
`
`[0081] Given the definition of FE it is advantageous if the
`following criterion holds:
`
`SEE0(13, 4 rim)
`E>T
`SEE(13, 4 rim)
`
`SEE0(13, 4 rim)
`E >T
`SEE(13, 4 rim)
`
`with the filter efficiency FE defined as
`
`250nm
`SEE0(A)dA
`I
`20nm
`250nm
`mm SEE(A)dA
`
`_
`FE _
`
`the integral ratio of the SEE
`[0082] This means that
`covering a wave length range from 20 to 250 nm is greater
`than the same SEE evaluated at the reference wave length of
`13.4 nm.
`
`with an absorber gas, the SEE is defined as:
`SEE(}\.):I(}\.)'(1-/1()\.))'S()\.)
`
`without an absorber gas, the SEE is defined as:
`SEEo(7~):1(7~)'(1-Ao(?»))'5(?»)
`
`

`
`US 2008/0055712 Al
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`Mar. 6, 2008
`
`and
`
`l(}t):spectral intensity of the light source (incl. the reflec-
`tivity of the mirrors),
`A(}t):spectral intensity attenuation along the light path 2
`with the absorber gas,
`AO(}t):spectral intensity attenuation along the light path
`without the absorber gas,
`S(}t):sensitivity of the photoresist on a wafer.
`6. Filter system according to claim 1, wherein the
`absorber gas having the property:
`
`SEE0 (A)
`SEE(/1) >
`
`for at least one wavelength in the range between 20 and 250
`nm and
`
`with an absorber gas, the SEE is defined as:
`SEE()\.):IO\.)'(1-/1()\.))'S(}\.)
`
`without an absorber gas, the SEE is defined as:
`SEEo(7~):1(7~)'(1-Ao(?»))'5(?»)
`
`and
`
`l(}t):spectral intensity of the light source (incl. the reflec-
`tivity of the mirrors),
`A(}t):spectral intensity attenuation along the light path 2
`with the absorber gas,
`AO(}t):spectral intensity attenuation along the light path
`without the absorber gas,
`S(}t):sensitivity of the photoresist on a wafer.
`7. Filter system according to claim 1, wherein the
`absorber gas comprises at least one of the group of phos-
`phine, benzene, carbon disulfide, ethylene, helium, hydro-
`gen, neon, argon, krypton and xenon.
`8. Filter system according to claim 1, wherein the
`absorber gas is introduced between the light source and a
`first mirror of a lithography system.
`9. Filter system according to claim 1, wherein the
`absorber gas is a gas stream intersecting the light path.
`10. Filter system according to claim 1, wherein the
`absorber gas flows with a drift velocity which is at least as
`high as the thermal velocity of molecules in one direction in
`the absorber gas given by
`
`RT
`Vzhermal = T -
`
`11. Filter system according to claim 1, wherein the cone
`angle defined by
`
`v
`
`coneangle = 2 >1: arctam[
`
`rhgrmal]
`Vdnfi
`
`is less than 45°, especially less than 20°.
`12. Filter system according to claim 1, wherein the
`absorber gas is recycled after the absorption process.
`13. Filter system according to claim 1, wherein the
`absorber gas is cooled after it passed through the light path.
`14. Filter system according to claim 1, wherein the light
`path passes at least partially through a vacuum.
`
`15. Filter system according to claim 1, wherein the
`absorber gas is filtered after it passed through the light path.
`16. Filter system according to claim 1, wherein the
`absorber gas flow is perpendicular to the light path.
`17. Filter system according to claim 1, wherein the
`absorber gas flow is maintained by a pump system.
`18. Filter system according to claim 1, wherein the
`absorber gas flow is pulsed, especially pulsed in synchrony
`with the light source.
`19. Filter system according to claim 1, wherein the
`flowing absorber gas is ionized and collector means com-
`prise electrical means for collecting an ionized absorber gas.
`20. Filter system according to claim 1, wherein the
`absorber gas flow is a free flow intersecting the light path.
`21. Filter system according to claim 1, wherein the free
`flowing absorber gas is collected by collector means after
`intersecting the light path.
`22. Filter system according to claim 1, wherein the free
`flowing absorber gas is collected by catch plates.
`23. Filter system according to claim 1, wherein the
`absorber gas is flowing through at least one filter chamber
`positioned in the light path.
`24. Filter system according to claim 1, wherein the at least
`one filter chamber comprises a container at least partially
`made of beryllium.
`25. Filter system according to claim 1, wherein at least
`two of the filter chambers contain different absorber gases.
`26. Filter system according to claim 1, wherein the light
`path is going through at
`least one chamber filled with
`absorber gas at pressure level different from the surrounding.
`27. Filter system according to claim 1, wherein at least
`one pump is connected to the at least one chamber to control
`the pressure level in said at least one chamber.
`28. Filter system according to claim 1, wherein at least
`one pump is connected to a plurality of chambers, each of
`the chambers having a different pressure level
`than the
`neighboring chamber.
`29. Filter system according to claim 1, wherein the light
`source is a plasma source for the use in an EUV lithography
`process, in particular a light source generated by a laser.
`30. Filter system according to claim 1, wherein the
`semiconductor device is at least one of the groups of volatile
`memory chip, non-volatile memory chip, DRAM memory
`chip and microprocessor.
`31. Filter system according to claim 1, wherein the
`absorber gas is distributed throughout a closed lithography
`equipment.
`32. Filter system according to claim 1, wherein the
`absorber gas is selectively removed from certain parts of the
`lithography equipment, in particular mirrors.
`33. Filter system according to claim 1, wherein absorber
`gas is recycled and filtered by a filter unit.
`34. Lithography apparatus for processing semiconductor
`substrates, comprising
`a light source capable of emitting EUV light;
`a plurality of mirrors for directing the light to the semi-
`conductor substrate;
`means for introducing and removing the semiconductor
`substrates,
`whereby the path of the light passes through an area filled
`with an absorber gas.
`35. Lithography apparatus for processing semiconductor
`substrates with at least one filter system according to claim
`1.
`
`36. Use of a filter system according to claim 1 in the
`manufacturing of at
`least one of the groups of volatile
`
`

`
`US 2008/0055712 Al
`
`Mar. 6, 2008
`
`memory chip, non-volatile memory chip, DRAM memory
`chip and microprocessor.
`37. Method for filtering light emitted by a light source in
`a lithography process for the production of semiconductor
`devices with a flowing absorber gas for at least one wave-
`length 0») in the range between 20 to 250 nm, the flowing
`absorber gas intersecting the light path emitted by the light
`source.
`
`38. Method according to claim 1, wherein
`a) the light source provides a light beam for structuring a
`wafer, the light beam being reflected by a plurality of
`
`mirrors and at least one reflective mask, whereby a
`projected image is transferred from the mask to a resist
`on the wafer forming a pattern in the resist,
`b) the resist pattern is transferred into the wafer by an
`etching process,
`in particular a reactive ion etching
`process, followed by
`c) a removal of the resist.
`39. Semiconductor device manufactured by a method
`according to claim 1.
`
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