Patent Application
20070187272
This application claims priority to German Patent Application 10 2005 061 571.6, which was filed Dec. 22, 2005 and is incorporated herein by reference.
The invention relates to a device for the storage and use of at least one photomask for lithographic projection and a method for using the device in an exposure installation.
For the production of integrated circuits, layers provided with different electrical properties are usually applied on semiconductor wafers and patterned lithographically in each case. A lithographic patterning step usually consists in applying a photosensitive resist, exposing the latter with a desired structure for the relevant plane and developing it, and subsequently transferring the resultant resist mask into the underlying layer in an etching step or using the resist mask as an implantation mask for altering the electrical properties of the underlying layer in a targeted manner.
For the lithographic projection step of a circuit pattern, a wafer scanner or wafer stepper is usually used as exposure apparatus. In the exposure apparatus, the photosensitive resist layer is exposed with electromagnetic radiation having a predetermined wavelength, which, in present-day exposure technologies, lies for example in the UV or DUV range at 256 nm, 193 nm or 157 nm. The exposure dose present during the exposure of the photosensitive resist layer at the location of the semiconductor wafer is chosen according to the specifications of the resist layer.
Each individual layer of the circuit pattern is usually transferred to the semiconductor wafer by means of a photomask. The photomask comprises a transparent substrate layer provided with absorbent elements, such as, e.g., a chromium layer, which simulate the circuit pattern. The photomask, also called a reticle, is often provided with a protective film (pellicle). The protective film serves to protect the structure side of the transparent substrate layer from deposits. Deposits on the protective film itself are not normally transferred to the resist layer during lithography since the protective film lies outside the focal range for imaging onto the resist layer.
In large-volume fabrication processes, diverse attempts are made to optimize the productivity. Besides the miniaturization of structure dimensions on the semiconductor wafers and the provision of process installations for semiconductor wafers having a diameter of 300 mm, a time-saving handling of the semiconductor wafers and photomasks in the process installations is also an important optimization variable.
In order to be able to use process installations from different manufacturers or of different types, standardized equipment is typically used. Thus, by way of example, the standardization committee “SEMI” standardizes a multiplicity of equipment for the semiconductor industry with regard to the interoperability thereof.
SEMI Standard 111-0304 defines the configuration of reticle containers which are fed to the lithographic projection installations via a defined interface. Reticle containers which satisfy said standard are usually referred to as reticle SMIF pod (SMIF=standard mechanical interface) or by the abbreviation “RSP”, the purpose of which is to enable the reticles to be stored and transported within wafer fabrication. Reticle containers in accordance with said standard have a housing and a plate which is arranged at the bottom of the housing and which can be automatically closed or opened by the lithographic projection installations or reticle inspection systems. In this case, RSPs comprising one reticle and also RSPs comprising six reticles are used as storage containers.
In the case of the photomasks used in lithographic exposure processes, particles or contaminations can attach to the surface by adhesion from the surrounding atmosphere. Thus, by way of example, the presence of ammonium ions and/or sulfate ions on the reticle surface leads to the formation of ammonium sulfate ((NH4)2SO4) or to the formation of ammonium oxalate ((NH4)2C2O4H2O). These crystals can grow with energy being radiated in by the light source of the exposure apparatus.
An example of crystal growth and irradiation with UV light is described below. Air normally contains hydrogen sulfide (H2S) in a low concentration. Together with oxygen, sulfur dioxide forms in accordance with the reaction equation:
2H2S+3O2−>2SO2+2H2O. [1]
With light being radiated in during the lithographic projection, free oxygen radicals are formed which react with sulfur dioxide in accordance with the following reaction equation:
SO2+O−>SO3 [2]
Together with (residual) water from the air, aerosol particles arise, which are chemically stable, in accordance with the following reaction equation:
SO3+H2O−>H2SO4 [3]
In the presence of impurities, in this case ammonia, said aerosol particles react to form ammonium sulfate, in accordance with the following reaction equation:
H2SO4+2NH3−>(NH4)2SO4 [4]
Photomasks in exposure apparatuses having exposure wavelengths in the DUV range exhibit a growth of said crystals which takes place virtually like an avalanche. Consequently, the photomasks have to be regularly monitored and cleaned.
This cleaning is usually carried out at the mask company by the manufacturer of the photomasks. For cleaning purposes, the photomasks are introduced into an acid bath. By way of example, a solution containing sulfuric acid is used as the acid bath. However, it has been found that the surface of freshly cleaned photomasks is still relatively susceptible to crystal growth.
In addition to the productivity stoppage and the high costs due to the cleaning, it can also occasionally happen that a photomask is destroyed or damaged during cleaning. Furthermore, by way of example, phase shifter masks can only be cleaned a few times since the properties of the phase shifters can change.
According to an embodiment of the present invention, a device for the storage and use of at least one photomask for lithographic projection is provided, which comprises a container suitable for receiving a photomask. The container includes a container housing and a closable opening device situated at the container housing and serving for the entry and issuing of the photomask. The container has at least one gas inlet opening arranged in such a way that, in the case of purging the photomask, a purge gas flushes around the photomask with a laminar flow.
According to this embodiment of the invention, impurities directly in the vicinity of the photomask in the volume over the transparent substrate are removed by means of the chemically active gases or gas mixtures, thereby suppressing crystal growth on the surface of the photomask. Consequently, the photomask can be used significantly longer in a lithographic exposure process without suffering from the depositing of particles.
According to a further embodiment of the present invention of a method for using the device is provided. A fabrication installation includes at least one exposure apparatus suitable for receiving the photomask. The photomask is fed from the container into the exposure apparatus. One or more exposure processes are carried out with the exposure apparatus using light from a UV source. The photomask is removed from the exposure apparatus into the protective container and the photomask is cleaned in the container by purging with the purge gas.
In a further embodiment, the following steps are furthermore performed. A microwave source is provided. The photomask is irradiated with microwave radiation. The photomask is cleaned in the container by purging with the purge gas.
In a further embodiment, the following steps are furthermore performed. An infrared source is provided. The photomask is irradiated with infrared radiation and the photomask is cleaned in the container by purging with the purge gas.
In accordance with one embodiment of the invention, the device is used in an exposure apparatus which can be operated with conventional loading and unloading stations and also storage containers.
The invention will now be explained in more detail with reference to the accompanying drawings:
The invention is explained below in the context of the lithographic patterning of semiconductor wafers, which is carried out, for example, during the fabrication of microelectronic circuits. It is appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to apply the method and to apply the device of the invention, and do not limit the scope of the invention. Accordingly, the invention can be applied in a multiplicity of production technologies in which a patterning step is effected by means of a photomask, thus, for example, during the production of thin-film components, such as, e.g., TFT elements, or else in nanotechnology.
Depending on the type of mask, the structure elements 16 are embodied in absorbent, partly absorbent or phase-shifting fashion. The mixture of these properties of the structure elements 16 is likewise possible. Absorbent structure elements 16 usually comprise chromium or black chromium. A thinned chromium or molybdenum silicide is used for partly absorbent structure elements 16. The phase-shifting properties of the structure elements 16 may be achieved for example by the use of molybdenum silicide or by an etch into the mask substrate 10 in order to form trench-like structure elements. It is, of course, also possible to use reflective mask types such as are used in EUV lithography by way of example.
As is shown in
A protective film 20, also called pellicle, is furthermore fixed on the frame 18, for example by adhesive bonding, above the transparent substrate 10. Said protective film 20, together with the front side 12 of the transparent substrate 10 and the side walls of the frame 18, forms an at least spatially closed-off volume 26.
The protective film 20 is permeable to gaseous substances of the purge gas toward the volume 26. For this purpose, the protective film 20 may be provided with a pinhole, for example, in order to enable a gas exchange between the volume 26 and the region outside the volume 26. Other openings or cutouts are likewise conceivable.
The device according to the invention furthermore has a container 30. The container 30 has a container housing 32 and a bottom flap 34 situated at the container housing 32 as a closeable opening. Other openings might be provided for example at the sidewalls of the container 30. The container housing 32 has an electrostatically dissipating coating, whereby the use of ionizers or the like is obviated.
The bottom flap 34 is provided for the entry and issuing of the photomask 5. Furthermore, the bottom flap 34 has a gas inlet opening 40 and a gas outlet opening 42, for example in the form of circular holes within the bottom flap 34. In this case, the position of the gas inlet opening 40 and of the gas outlet opening 42 within the bottom flap 34 can be chosen freely.
The container 30 is embodied so as to be able to receive the photomask 5. For this purpose, the container housing 32 is provided with a reticle holder 50 in order to hold the photomask 5 in a predetermined position. The reticle holder 50 may be embodied as a pin or clamp, for example, thereby preventing the photomask 5 from slipping in a lateral direction and upward within the container housing 32. Furthermore, the bottom flap 34 has a reticle support 52 in order to hold the photomask 5 in a predetermined position. The reticle support 52 is intended to prevent the photomask 5 from slipping in the direction of the bottom flap 34 within the container housing 32.
For simpler use of the device in accordance with
As mentioned in the introduction, for time-saving handling within semiconductor fabrication installations, a standardization of the process equipment used is provided. In consequent fashion, the container 30 and the bottom flap 34 are embodied in accordance with an industry standard, for example according to the SEMI Standard. The mechanical interface likewise corresponds to an industry standard according to one of the SEMI Standards. It goes without saying that it is likewise conceivable to use standards appertaining to a different industry standard or else proprietary, in-house specifications.
The device in accordance with this embodiment furthermore has a purge device 44. The purge device 44 serves to remove impurities by means of the purge gas in the volume 26. As mentioned in the introduction, said impurities are for example ammonia, carbon dioxide or else sulfur-containing gases, such as, e.g., hydrogen sulfide or sulfur dioxide. Said impurities could cause crystallization on the photomask 5, which is prevented on account of the impurities being transported away by means of the purge gases of the purge device 44.
For this purpose, as shown on
Particular consideration is given in this case to the position of the gas inlet opening 40 and the gas outlet opening 42. In order to prevent the purge gas from flushing around the photomask 5 in a turbulent flow, the gas inlet opening 40 and the gas outlet opening 42 are arranged offset with respect to the photomask. This results in a flow that is as laminar as possible. The precise position of the gas inlet opening 40 and of the gas outlet opening 42 can also be determined by means of a computer simulation.
As is shown in
The purge device 44 is preferably embodied with a mixing device for purging with a plurality of gases. For this purpose, a mixing device comprising a T-piece and a mixing valve may be provided in order to admix the plurality of gases in a desired mixing ratio. Moreover, in addition to the mixing ratios, the gas temperature and/or the gas partial pressures of the purge gas can be controlled by means of the purge device 44. The container 30 is shown together with the purge device 44 in
A plurality of purge gases is appropriate for eliminating the impurities. In accordance with reaction equations [1] to [4], crystal growth is prevented by removing ammonia or sulfur-containing gases. Generally, a gas mixture for cleaning organic contaminations or a gas mixture for cleaning inorganic contaminations may be fed as purge gas. The purge gas used may also be weakly chemically active, in which case, in particular, the protective film 20 should not be attacked by the purge gas.
By way of example, a nitrogen-ozone gas mixture is provided for cleaning organic contaminations; the cleaning of inorganic contaminations is effected, e.g., by means of a nitrogen-argon gas mixture. It is likewise conceivable to feed a nitrogen-hydrogen gas mixture, for example as a 90% nitrogen and 10% hydrogen forming gas. A nitrogen-carbon dioxide gas mixture is likewise possible.
The impurities within the container are transported away efficiently on account of the purging with the above-mentioned purge gases, thereby lengthening the interval between external cleaning steps in a mask company. As a result, the service life of the photomask is significantly lengthened, and corresponding cleaning costs are saved.
A further aspect is that water is a starting point for many crystallization processes on the photomask 5. Therefore, provision is made for choosing the purge gas such that water molecules are removed from the photomask 5. Water molecules often occur as a molecular monolayer on the surface of the transparent substrate. By purging with a dried air mixture, a so-called XDA gas (XDA=extremely dry air), from which the water component has been removed to the greatest possible extent, the partial pressure is shifted correspondingly, resulting in evaporation of the water on the surface of the photomask.
As is shown in
In order to be able to receive a multiplicity of different containers, the purge device 44, rather than being connected to the bottom flap 34 directly, is now connected via an adapter plate 58 arranged below the bottom flap 34 of the container. In this case, the adapter plate 58 is introduced into the holding frame 56. For this purpose, the holding frame 56 has fixings 60 at its edge in order to fix the adapter plate 58.
The adapter plate 58 is embodied in the holding frame 56 as a rack bottom on which the container 30 with the photomask 5 can be placed. The fixings 60 are embodied in screwable fashion in order to enable the adapter plate 58 to be exchanged. The gas feed line 46 is shown as a part of the adapter plate in
For receiving the container 30, a hinged mechanism 64 is provided in the holding frame 56, which mechanism secures the container 30. As shown in
To summarize, the holding frame 56 is connected to the purge device 44, then, so that purging with the purge gases can be carried out during the storage of photomasks. The combination of container 30 and holding frame 56 may be embodied in a manner conforming to an industry standard in this case, for example according to the SEMI standard.
This concept is extended below, with reference to
A further embodiment is described making reference now to
In
Both, the gas inlet opening 40 and the gas feed line 46 can be arranged as stubs having respective diameters adapted to house the gas inlet opening 40 within the gas feed line 46 or vice versa.
In
The upper part of the gas feed line 46 has a diameter larger than the size of the cylindrical stub of gas inlet opening 40. Accordingly, the gas inlet opening 40 and the gas feed line 46 can be put together in an overlapping manner. It should be noted that many different shapes and cross-sections of the gas inlet opening 40 and the gas feed line 46 can be used, including but not limited to elliptical or rectangular shaped structures having conical, partially conical or multi-step cross-sections.
In order to prevent gas from leaking at the joint between the gas inlet opening 40 and the gas feed line 46, a gasket 36 is introduced in-between. As shown in
The gasket 36 can be of toroidal or ring-like shape as depicted in
In order to achieve easily disassembling of the container 30 to the gas system 44, the gasket can be provided as a collapsible gasket having a passive state of being either collapsed or expanded. By applying outside pressure, the state of the collapsible gasket can be selected.
As an example, self inflatable synthetics can be used which seal the gas inlet opening 40 and the gas feed line 46 while inflated. Before removing the container 30 from the gas system 44, extraction by suction or by applying a vacuum collapses gasket. During purging of gases, no action has to be taken, as the self inflatable gasket provides a seal and clamps the gas inlet opening 40 and the gas feed line 46.
In another conceivable arrangement of the gasket is shown in
The gasket 36 is arranged as a sleeve having a double-walled cross-section. The double-walled cross-section results in an inner wall member 37 and an outer wall member 38 joined at cusp 39. As shown in
Generally speaking, the cusp 39 should point in the same direction with respect to the gas flow purged through the gas feed line 46. This results in a self-controlled sealing while purging gas through the gas inlet opening 40. As the incoming gas flow presses against the inner wall member 37 and outer wall member 38, a force results which tends to open the inner wall member 37 and outer wall member 38 with respect to the cusp 39. Accordingly, the inner wall member 37, or generally speaking the free movable member of gasket 36, is pressed against the inner wall of the stub of gas inlet opening 40. As long as gas flow is maintained through the gas feed line 46, the gasket 36 seals the joint between the gas inlet opening 40 and the gas feed line 46. In addition, the gasket provides some clamping of the gas inlet opening 40 and the gas feed line 46 due to the pressure created by the purging gas.
In order to provide a seal between the gas inlet opening 40 and the gas feed line 46, an elastic material can be used for the inner wall member. It is also conceivable to fabricate the entire gasket using an elastic material such as rubber or the like. Alternatively, the gasket can be formed using a stiff material, e.g., a synthetic material, being subdivided into a plurality of fins which partially overlap each other. The fins are arranged such that the purge gas bends the fins along the cusp 39 in order to seal the joint between the gas inlet opening 40 and the gas feed line 46.
It should be noted that a further gasket can be arranged in cases when the container 30 has a gas outlet through which the purge gas is removed by a gas discharge line. The further gasket is arranged between the gas outlet opening and the gas discharge line and provides a similar as described above a collapsible seal between the gas outlet opening and the gas discharge line. In order to allow an unfolding by the passing gas stream, the cusp 39 joining the inner wall member 37 and the outer wall member 38 has to be oriented in the opposite direction so that the discharged gas unfolds the sleeve formed by the inner wall member 37 and the outer wall member 38. Accordingly, the gas outlet opening encapsulates the gas discharge line which can be arranged as a stub, similar to the embodiment of
In a further embodiment, sealing can be archived with a magnetic enforced gasket. This could be archived with a metal or permanent magnet in the one interface side and electro magnet enforcement at other side (not shown in
According to the embodiments described with respect to
This concept can be extended to the handling of wafer pods within automated stockers. There, container 30 carries a plurality of semi-conductor wafers instead of photomask 5. In a semi-conductor manufacturing unit, the wafer pods are frequently transported between a shelf and automatic processing units. Here, a sensor at the shelf operated by a robot stops gas purging and initiates gasket collapsing for placement and lifting of the container 30. At the processing units, a placement sensor will open the gas valve in order to expand the gasket. In case of purging the semiconductor wafer during processing or storage, it is not necessary to interrupt the gas flow because there is no gas leakage and accordingly no human risk present.
The device described previously is able, by means of the purge device 44, to effectively purge impurities within the container 30. On account of relatively long periods of use, however, it can happen that crystals have nevertheless formed on the transparent substrate 10 or the structure elements 16. In order to be able to effectively remove them, a description is given below of an activation of the crystals by means of electromagnetic radiation, which can be employed in addition to the measures already mentioned.
As is shown in
The microwave source 80 may for example also be integrated into the above-described storage system 70 in accordance with
In the case where the microwave source 80 is integrated into the storage system 70, the bottom flap 34 is open in this case in order to allow the microwave radiation to impinge on the photomask 5 unimpeded. In this case, the microwave radiation irradiates the surface of the transparent substrate 10 and also the structure elements 16 thereof from the side of the protective film 20. The wavelength and/or intensity of the microwave radiation of the microwave source 80 is chosen such that chemical bonds of deposits 24 on the surface of the transparent substrate 10 can break up. It goes without saying that it is also possible to remove deposits in the volume 26 between the surface of the transparent substrate 10 and the protective film 20 or on the surface of the protective film 20 itself. The deposits 24 on the surface of the transparent substrate 10 are usually disruptive for lithographic projection.
If the deposits comprise ammonium sulfate, the wavelength and/or intensity of the microwave radiation of the microwave source 80 should be chosen such that hydrogen-oxygen bonds break up. This will be explained in more detail again with reference to
As is known, bonds within water or OH components are excited by microwave radiation, and they may break up in the process. Consequently, an equilibrium arises between the starting product (NH4)2SO4 on the one hand, and the volatile constituents SO3, NH3 and H2O, on the other hand, which equilibrium is shifted toward the volatile constituents in the case of irradiation with microwaves. The formation of particles on the photomask 5 is thus prevented as a result of the breaking up of the OH bonds 88 according to the structural formula of ammonium sulfate in accordance with
In this case, the volatile constituents are entrained by the purge gas, thereby preventing renewed crystallization. As is shown in
The wavelength of the microwave radiation of the microwave source 80 may be chosen such that it lies within a range in which the protective film 20 is as transparent as possible to microwave radiation. In the case of a protective film made of a Teflon-containing material, the transparent range of the frequency of the microwave radiation lies between 2 GHz and 3 GHz. Consequently, it is possible to use a conventional microwave source having a frequency of the microwave radiation of approximately 2455 MHz, such as is used e.g. in a microwave oven.
In order to bring the microwave radiation of the microwave source 80 into the transparent range of the protective film 20, it is possible to provide a filter 82 comprising, e.g., the material of the protective film 20. Consequently, only radiation having a frequency at which the protective film 20 is transparent advances as far as the photomask 5.
In a further embodiment, the microwave source 80 may also be pulsed in order as far as possible to prevent electrostatic charging or spark-overs on electrically conductive structure elements on the transparent substrate. Likewise, the microwave radiation should not damage adhesives for the fixing of the frame 18 or the protective film 20.
As is shown in
The infrared source 90 heats crystalline deposits on the surface of the transparent substrate in order that their volatile constituents are subsequently removed by means of the purge gas. In this case, the gas feed line 46 and the gas discharge line 48 are once again arranged in direct proximity to the photomask 5 in order to remove the volatile constituents.
The wavelength of the infrared radiation of the infrared source 90 is once again chosen such that it lies within a range in which the protective film 20 is at least partly transparent to infrared radiation, but the impurities are activated to a sufficient extent. This necessitates, on the one hand, a high transmission for infrared radiation in the protective film 20 and a high absorption for infrared radiation of the impurities.
A useable frequency range is explained below with reference to
According to the invention, impurities directly in the vicinity of the photomask are removed by means of purge gases, thereby suppressing crystal growth on the surface of the photomask. Accordingly, the time between photomask cleaning steps can be significantly lengthened.
Having described embodiments for a device for the storage and use of at least one photomask for lithographic projection and a method for using the device in a fabrication installation and non-volatile memory cells, it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as defined by the appended claims.
Having thus described the invention with the details and the particularity required by the patent laws, what is claimed and desired to be protected by Letters Patent is set forth in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 061 571.6 | Dec 2005 | DE | national |
