Photocopying – Projection printing and copying cameras – With temperature or foreign particle control
Reexamination Certificate
2002-11-12
2004-12-07
Mathews, Alan (Department: 2851)
Photocopying
Projection printing and copying cameras
With temperature or foreign particle control
C355S053000, C356S237400, C250S492200
Reexamination Certificate
active
06829035
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to semiconductor manufacturing processes, and specifically to methods and apparatus for cleaning and handling of lithographic masks used in producing semiconductor devices.
BACKGROUND OF INVENTION
As the trend continues to reduce the size of semiconductor devices, optical lithography using conventional transmission masks, such as chrome on glass (COG) or phase shift (PSM) masks, will no longer suffice as a viable technique for printing advanced devices on semiconductor wafers. Transmission lithography has been extended to ever shorter wavelengths, down to 157 nm in the far ultraviolet (UV), in order to reduce the size of device features. However, the still shorter wavelengths necessary for printing even smaller device structures are readily absorbed in transmission materials. Alternative technological candidates to replace optical lithography include: electron projection lithography (EPL) and an all-reflective technology called extreme ultra-violet lithography (EUVL).
Virtually all masks used in production today employ a pellicle to protect the mask surface from particulate contamination. The pellicle is a relatively inexpensive, thin, transparent, flexible sheet, which is stretched above and not touching the surface of the mask. Pellicles provide a functional and economic solution to particulate contamination by mechanically separating particles from the mask surface. The mask is transported and used for lithographic exposure with the pellicle in place. When a mask is used for exposure, with the pellicle in position above the mask, only the details of the mask's focal plane itself are printed. Particulate material located on the pellicle surface is maintained outside of the focal plane of projection. As a result, particulate material is not printed. When the pellicle eventually becomes damaged or too dirty to use, the mask is removed to a workshop, and the pellicle is replaced.
A suitable pellicle material and structure have yet to be defined for 157 nm technology. The options to date include using either no pellicle or a very expensive hard pellicle. An inexpensive soft pellicle that is capable of withstanding multiple exposures to 157 nm light has yet to be developed. It appears, therefore, that masks for lithography at 157 nm and for shorter wavelengths must be used without the protection of a pellicle. If a no-pellicle option is chosen, the masks must be cleaned frequently, and the cleaning technique must be suitable for multiple cleaning cycles without inducing any significant damage to sensitive mask films. Most contaminants absorb radiation at short wavelengths, and it is therefore imperative that the mask surface be completely free of any contamination that may absorb radiation.
Not only must mask particle contamination removal efficiency be increased, but the minimum particle size to be removed must also decrease. For example, in EUV lithography, masks must be cleaned to remove particles as small as 70 nm, since particles of this size are already printable at EUV lithography wavelengths. Conventional cleaning technologies such as sulfuric-peroxide mixture (SPM) and standard cleans (SC-1 and SC-2) do not fulfill all of the previously mentioned contamination removal criteria. If these conventional cleaning procedures must be applied to the mask repeatedly (due to the absence of a mask pellicle), they are likely to cause rapid degradation of delicate mask film layers.
Various methods are known in the art for stripping and cleaning foreign matter from the surfaces of semiconductor wafers and masks, while avoiding damage to the surface itself. For example, U.S. Pat. No. 4,980,536, whose disclosure is incorporated herein by reference, describes a method and apparatus for removal of particles from solid-state surfaces by laser bombardment. U.S. Pat. Nos. 5,099,557 and 5,024,968, whose disclosures are also incorporated herein by reference, describe methods and apparatus for removing surface contaminants from a substrate by high-energy irradiation. The substrate is irradiated by a laser with sufficient energy to release the particles, while an inert gas flows across the wafer surface to carry away the released particles.
U.S. Pat. No. 4,987,286, whose disclosure is likewise incorporated herein by reference, describes a method and apparatus for removing minute particles (as small as submicron) from a surface to which they are adhered. An energy transfer medium, typically a fluid, is interposed between each particle to be removed and the surface. The medium is irradiated with laser energy and absorbs sufficient energy to cause explosive evaporation, thereby dislodging the particles.
Various methods are known in the art for discriminating and localizing defects on substrates. U.S. Pat. Nos. 5,264,912 and 4,628,531, whose disclosures are incorporated herein by reference are examples. Foreign particles are one type of defects that can be detected using these methods.
U.S. Pat. No. 5,023,424, whose disclosure is incorporated herein by reference, describes a method and apparatus using laser-induced shock waves to dislodge particles from a wafer surface. A particle detector is used to locate the positions of particles on the wafer surface. A laser beam is then focused at a point above the wafer surface near the position of each of the particles, in order to produce gas-borne shock waves with peak pressure gradients sufficient to dislodge and remove the particles. It is noted that the particles must be dislodged by the shock wave, rather than vaporized due to absorption of the laser radiation. U.S. Pat. No. 5,023,424 further notes that immersion of the surface in a liquid (as in the above-mentioned U.S. Pat. No. 4,987,286, for example) is unsuitable for use in removing small numbers of microscopic particles.
SUMMARY OF INVENTION
It is an object of some aspects of the present invention to provide improved methods and apparatus for removal of microscopic particles from lithographic masks used in semiconductor device production. In the context of the present patent application and in the claims, the word “particle” is used broadly to refer to any contaminant or other foreign substance that must be removed from the mask surface.
In embodiments of the present invention, a lithography tool, for use in producing semiconductor devices, comprises one or more lithography stations, together with a mask cleaning station. The lithography and mask cleaning stations are contained in a common enclosure, and a robot is preferably used to transfer the masks between the cleaning and lithography stations in order to isolate the mask and the stations from ambient air and from human contact. This arrangement is particularly advantageous in dealing with masks without pellicles, since it allows particles to be removed frequently from the masks, in the production environment, without removing the masks to a separate mask shop. This arrangement facilitates the higher level of mask cleanliness that is required for far UV and EUV lithography.
Preferably, the lithography tool also comprises an inspection station, which checks each mask before or after use to verify that the mask is still clean and, if not, to determine the locations of any contaminant particles on the mask. If the inspection station finds the mask to be contaminated, the robot passes the mask to the cleaning station. Based on coordinates of the particles determined by the inspection station, the cleaning station applies a local cleaning process to remove the particles. Preferably, the local cleaning process involves wetting the particle location with a suitable fluid, and then irradiating the location with laser radiation, most preferably UV laser radiation. This cleaning approach gives optimal removal of contaminant particles, without affecting in any way the remainder of the mask.
Alternatively, various other local cleaning methods may be used in conjunction with the inspection station. Examples of such methods include localized plasma application; local application of pressurized
Applied Materials Israel, Ltd.
Blakely , Sokoloff, Taylor & Zafman LLP
Mathews Alan
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