Patent Application
20070037067
The invention relates to optical pellicles for photomasks used in photolithographic manufacturing. In particular, the invention relates to optical pellicles that may have pressure-equalization orifices formed within the pellicle frame.
The manufacture of semiconductor devices typically involves applying a layer of a photosensitive substance (a photoresist) to the surface of a target wafer. The photoresist is exposed to light in a selected pattern using a photomask, and the photoresist is then developed to leave exposed regions of the wafer. Typically, the exposed regions are subsequently etched away or otherwise modified, and the residual photoresist is removed. The pattern of the photomask typically possesses extremely fine details, and the presence of even tiny particles on the surface of the photomask can interfere with the accurate reproduction of the pattern on the target wafer.
To minimize particulate contamination at the mask surface, optical pellicles have been developed that protect the photomask. An optical pellicle is a frame-mounted transparent membrane that is attached to the photomask surface, so that contaminating particles fall onto the pellicle membrane and not the surface of the photomask. The pellicle frame holds the pellicle membrane at a sufficient distance above the mask surface so that any particles that may fall upon the membrane lie outside the focal plane of the illuminating light. As a result, these particles do not interfere with the projected mask pattern. The use of optical pellicles in semiconductor manufacture has helped mitigate the effects of contamination by dust and other particulates, and is widespread in the industry.
Pellicles may be packaged such that a protective cover is applied to the pellicle frame, opposite the side of the pellicle. This may create an airtight package with the pellicle, the pellicle frame, and the protective cover. Alternatively, a pellicle may be shipped already adjacent to the photomask, also forming an airtight package.
Pellicle packages are typically airtight to reduce contamination from dust and other contaminate particles. A problem occurs, however, when temperature and pressure change during shipping. This may cause the packaging and/or pellicle to expand and/or contract and pellicles may be damaged as a result. Therefore, pressure equalization orifices, or vents, have been designed in the frame, in the adhesive layer of a pellicle configuration, and even in the photomask itself. Some of these configurations have utilized filters to allow for pressure equalization without damaging the pellicle and without allowing contaminate particles to enter the area between the pellicle and photomask.
After a pellicle with a protective cover is received by a pellicle user the protective cover is typically removed and a photomask is mounted to the pellicle. Even before the pellicle membrane, protective cover or photomask are attached to the pellicle frame, however, the pellicle user may need to clean the pellicle frame in order to remove any contaminate particles that may be present on the frame. For pellicles utilizing one or more filters, the portion of the equalization orifice between the filter and the area to be bounded by the frame, the pellicle membrane, and photomask may be the most important portion to clean. This is because these surfaces are very close to where the photomask will be located, and any contaminate particles in this portion will have already passed through the filter section and will no longer be shielded from the photomask itself once the photomask is applied. Therefore, in order to facilitate this cleaning it may be ideal to use a pellicle employing an equalization orifice and a filter, where the filter is located very near to the area that will be bounded between the pellicle membrane, the pellicle frame, and the photomask.
Accordingly, it is an object of the present invention to develop a system for equalizing the pressure when a pellicle with a protective cover is shipped by air or is otherwise exposed to conditions that cause ambient pressure to increase or decrease. Specifically, it is an object of the present invention to develop a system that equalizes this pressure while allowing for easy cleaning of the pellicle frame before a pellicle membrane is mounted to the frame and before the pellicle is either mounted to a photomask or to a protective cover.
The present invention provides an optical pellicle frame that includes an exterior surface and an interior surface, at least one vent extending through the frame, from the exterior surface to the interior surface, and a filter, wherein the filter is located within the vent and is at least three times closer to the interior surface of the frame than to the exterior surface of the frame.
Another aspect of the invention provides an optical pellicle that includes a pellicle frame and a pellicle membrane configured to cover the top surface of the frame. The pellicle frame includes an exterior surface, an interior surface, a top surface, and a bottom surface, at least one vent extending through the frame from the exterior surface to the interior surface, and at least one filter located within the vent, wherein the cross-sectional area of the pellicle frame at the location of the vent is configured in a U-shape,
so that no continuous surface exists from the top surface of the frame to the bottom surface for at least ¾ of the length of the frame spanning from the exterior surface of the frame to the interior surface.
Yet another aspect of the invention provides an optical pellicle that includes a pellicle frame, a pellicle membrane configured to cover the top surface of the frame, and a protective cover to which the bottom surface of the frame is mounted. The pellicle frame bounds an area between the frame, the pellicle membrane, and the protective cover, and the frame includes an exterior surface, a top surface and a bottom surface, at least one vent extending through the frame from the exterior surface to the bounded area, and at least one filter, wherein the distance from the exterior surface of the frame to the filter is at least three times the distance from the filter to the bounded area.
Before turning to the preferred embodiments, reference should first be made to
When optical pellicles are shipped to a customer, they typically are in the form of a pellicle membrane such as 2 mounted to a frame such as 4. Because they typically are not as yet mounted to a photomask, a protective plastic cover (not shown in
While the protective cover provides an effective seal to prevent particulate from depositing on the underside of the pellicle membrane, the airtight seal presents a problem when the cover-protected pellicle is transported by air to the customer's facility. Reduced ambient pressure will cause the pellicle membrane to bow outwardly as the atmospheric pressure captured in the space bounded by the pellicle membrane, frame and protective cover presses against the thin pellicle membrane. This may stretch or otherwise damage the pellicle membrane or adversely affect the bond between the membrane and the frame. Any such stretching or damage to the pellicle can be critical because it is important that a very high percentage of light pass directly through the pellicle membrane during photolithography, with very minimal reflection or refraction. Any distortion of the light could prevent accurate replication of the photomask pattern in the silicon wafer positioned therebelow during photolithography.
To prevent such damage to the pellicle membrane during shipping, a venting orifice channel such as that shown in
Other prior art attempts to solve this problem involve long channels (not shown in
However, after manufacture of the frame and before the frame is mounted to a pellicle membrane, a protective cover or a photomask, the frame must be cleaned in order to make sure that when a photomask is mounted, contaminate particulate will not fall onto the photomask and harm the photolithographic process. For a frame utilizing a filter, the surfaces between the filter and the area that will be bounded by the frame, a pellicle membrane and a photomask are the surfaces where cleaning is of the utmost importance. Any contaminate particulate in this area may no longer have any impediments between its current location and the photomask, when mounted. A pellicle may thus be designed so as to allow for filtered pressure equalization while allowing for easy cleaning of the pellicle frame before its use with a photomask.
As noted above, the use of optical pellicles in semiconductor manufacture has helped mitigate the effects of contamination by dust and other particulates, and use of such pellicles is widespread in the industry.
Referring to
Also as shown in
The pellicle frame 12 is typically made of a plastic or aluminum, although other suitable materials may also be used. The pellicle frame, and the vent defined therein, may be formed in varying ways. Typically, this forming will be done by casting or molding. Furthermore, the vent 26 and other elements of the frame may be formed with machining techniques, such as drilling.
A finished optical pellicle 10 may often be shipped by air or by another method resulting in varying air pressures between the area bounded by an optical pellicle and its ambient surroundings, as previously discussed. The vent 26 will normally function to allow this area bounded by the pellicle frame 12, the pellicle membrane 14 and the protective cover 16 to remain at the same ambient pressure as the area surrounding the pellicle. Before, during, and after shipment, the filter 28 may function to keep contaminate particulate out of the bounded area.
However, as described above, before any structure is attached to the pellicle frame 12, the frame may need to be cleaned to rid the frame surfaces of any contaminate particulate that could impair the photolithographic process. Most importantly, the interior surface 24 of the pellicle frame 12 and the portion of the vent 26 that is located between the filter and the interior surface must be cleaned. The cleaning of this area is most important because any particulate that has reached these surfaces will often no longer be protected by the filter 28, which normally protects particulates from passing into this area. Contaminate particulates located in this area pose a great risk of falling onto the photomask and projecting onto the photoresist when the photomask is attached to the bottom of the pellicle frame 12. Any traditional method of cleaning a pellicle may be used, such as thoroughly wiping down the structures or blowing off any contaminate particulates from them. After cleaning and after the pellicle membrane and protective cover are mounted to the pellicle frame, the optical pellicle will typically be substantially clean and ready for the photolithographic process.
By locating the filter at least three times closer to the interior surface 24 of the pellicle frame than to the exterior surface 22, the most important area to be cleaned is therefore minimized, which may allow for easier and more effective cleaning. This, in turn, typically means that less particulate will fall onto the photomask, and it can therefore be expected that the number of defective silicon wafers formed during the photolithographic process will likewise decrease.
As is further depicted in
The vent 26 of the pellicle frame 12 will also often include a filter seat 34 for positioning the filter 28 inside the vent 26, as shown in
Also consistent with the short length of the second vent section 32 relative to the first vent section 30, the filter 28 will typically have a height that is at least twice the length of the second vent section 32. As illustrated in the figures, the filter height is measured from the filter surface closest to the bottom surface 20 of the frame to the filter surface closest to the top surface 18 of the frame. In some embodiments this ratio of filter height to second vent section length will be at least four to one, rather than at least two to one. Furthermore, the length of the first vent section will typically be twice the height of the filter.
The filter 28 utilized by the optical pellicle 10 for allowing the passage of air but preventing the passage of certain contaminate particulates will often be a membrane-type filter. MF Millipore filters (trade name) or Cellotate filters (trade name) sold by Millipore Corporation may be used in the optical pellicle 10 of the present disclosure. These filters may be permeable filters and often are made of the raw materials cellulose-mixed ester or cellulose acetate.
In most embodiments the cross-sectional shape of the vent 26 will be the same from the exterior surface 22 of the pellicle frame to the interior surface 24, although it may also have varying cross-sections.
A fifth embodiment of an optical pellicle 510 of the present disclosure is shown in
The disclosure set forth above may encompass multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. The subject matter of the inventions includes all novel and non-obvious combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. Similarly, where the claims recite “a” or “a first” element or the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. It is believed that the following claims particularly point out certain combinations and subcombinations that are directed to one of the disclosed inventions and are novel and non-obvious. Inventions embodied in other combinations and subcombinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to a different invention or directed to the same invention, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the inventions of the present disclosure.
