Abstract:
An apparatus according to the present invention comprises a membrane layer and a supporting supportive mesh, wherein the membrane layer and the supportive mesh form a monolithic structure that absorbs less than ten percent of EUV (Extreme Ultraviolet) light passing through it in which the membrane layer is composed of a metal-ceramic composite.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/932,017, entitled “A Monolithic EUV transparent membrane and support mesh and method of manufacturing same,” filed Jan. 27, 2014, which is incorporated by reference in its entirety herein for all purposes. 
    
    
     BACKGROUND 
     Nearly all of today&#39;s electronic devices rely on semiconductor components known as integrated circuits (ICs), which are most commonly manufactured through a process known as optical lithography. Lithography is a patterning method that uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical “photoresist”, or simply “resist,” on to a substrate, such as a wafer of semiconductive material. A series of chemical treatments are used to either engrave the exposure pattern into the substrate, or enable new material to be deposited in the desired pattern to be deposited onto the substrate. By repeating this process using different photomasks, layers of an integrated circuit are gradually formed on the substrate. In complex integrated circuits, a modern CMOS wafer may go through the photolithographic cycle up to 50 times. The state of the art in optical lithography, particularly the wavelength of the light being used, is a gating factor in the ability to make increasingly smaller features on ICs. 
     Generally speaking, optical lithography begins with cleaning step, where a wet chemical treatment is used to remove any contaminants from its surface. The wafer may then be dried by heating it to a temperature sufficient to drive off any moisture present on the wafer surface. A liquid or gaseous “adhesion promoter”, such as Bis(trimethylsilyl)amine (“hexamethyldisilazane”, HMDS), may then be applied to the wafer surface in order to promote adhesion of the photoresist. 
     Next, the wafer is coated with a layer of photoresist by spin coating. Photoresist describes a category of light-sensitive materials. In the case of a so-called “positive photoresist,” the exposure to light causes a chemical reaction in the photoresist, which makes the exposed photoresist soluble in a “developer” solution. (With a “negative photoresist,” unexposed regions are soluble in the developer.) Typically, a liquid solution of photoresist is dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thick layer. The wafer is then exposed to a pattern of intense light via a photomask, causing the exposed photoresist to become soluble. The exposed photoresist is then removed, exposing regions of the substrate corresponding to the pattern created by the photomask. 
     Photomasks may be protected from particle contamination, a significant problem in semiconductor manufacturing, by a thin transparent film called a pellicle, mounted to one side of the photomask. The pellicle is placed far enough away from the mask so that particles that land on the pellicle will be out of focus and therefore not transferred to the wafer. Consequently, the optical properties of the pellicle must be taken into account. 
     After the soluble photoresist is removed, a chemical etching agent may applied to remove the uppermost layer of the substrate in any areas that are not protected by photoresist. Alternatively or additionally, material may be deposited into areas not protected by the substrate. The remaining photoresist is removed and the substrate now with a pattern corresponding to the photomask on its surface. This process may then be repeated using a different photomask for each layer of the desired integrated circuit. 
     With the progress of technology and the reduction of the feature size, the wavelength of the exposure light had to be reduced several times. Currently, the 193 nm lithography combined with immersion and double patterning technology is the state of the art. 
     Shorter wavelength lithography, known as next generation lithography (NGL), has been studied in order to produce IC with even smaller features. NGL uses shorter ultraviolet light (157 nm), extreme ultraviolet (EUV) light (e.g. 13.5 nm), X-ray (0.4 nm), and the even shorter wavelengths of electron and ion beams. Due to its optical characteristics, EUV lithography is generally accepted as the natural extension of optical lithography and is currently the most promising NGL technology. However, to this day, research and development of EUV technology has cost several billion US dollars worldwide. A single EUV exposure tool costs about US$70 million. 
     While most other NGLs require one-fold image reduction membrane masks, EUVL uses masks with four-fold image reduction, which makes mask fabrication feasible with current technology. However, in abandoning 157 nm lithography, the industry has created a technological jump from 193 nm to 13.5 nm wavelength, creating complex challenges across the board. Therefore, EUVL technology includes EUV resist technology, EUV aligners or printers, and EUV masks, as well as metrology, inspection, and defectivity controls. 
     One important aspect to bear in mind is the fact that all available materials are strong absorbers of EUV light and no material is transparent enough to make use of refractive optics (e.g. lenses). Therefore, it is necessary to make use of reflective optics only (e.g. mirrors) in EUVL optical systems. 
     However, challenges are present in almost every aspect of EUVL technology. Some challenges are common to all NGL technologies, e.g. resist resolution and line-edge roughness (LER). Other challenges are unique to EUVL, e.g. resist outgassing owing to the EUVL high-vacuum environment. In the past 20 years the main topics of research in EUVL have been: source, optics, mask, multilayer coating, resist, metrology, reticle handling, defects, and contamination control. It is, for example, a critical task to create a defect-free EUVL mask. EUVL mask technology includes mask blank preparation and pattern fabrication. Additionally, because of the harsh environment necessary for EUV lithography, it has previously been difficult or impossible to produce suitable protective pellicles, strong enough to shield the photomask from debris while still being suitably transparent to EUV light. Thus, what is needed is an EUV transparent membrane that is resilient enough to withstand the harsh vacuum environment necessary for EUV lithography. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments in accordance with the present disclosure will be described with reference to the drawings, in which: 
         FIG. 1A  is a partial cross-sectional view of a EUV transparent membrane and supporting mesh in accordance with various aspects of the present monolithic EUV transparent membrane and supporting mesh. 
         FIG. 1B  is a top down view of a EUV transparent membrane and supporting mesh in accordance with various aspects of the present monolithic EUV transparent membrane and supporting mesh. 
         FIGS. 2A through 2I  are block diagram illustrations of a partial manufacturing process flow depicting aspects of various embodiments of the present EUV pellicle manufacturing method. 
     
    
    
     DETAILED DESCRIPTION 
     Pursuant to the patent laws of the United States, this detailed description, discusses various illustrative embodiments of an EUV transparent support mesh (“the present support mesh”) and methods of manufacturing the same, with reference to the accompanying drawings. These illustrative embodiments are provided, as required by law, in order to give persons having ordinary skill in the relevant art a full, clear, and concise description of the invention defined by the claims, and to enable such a person to appreciate and understand how to make and use, not only the described illustrative embodiments of the present support mesh and associated methods of manufacturing, but various other embodiments that fall within the conceptual scope of present support mesh and associated methods of manufacturing, despite those various other embodiments not being explicitly described herein. 
     Neither this detailed description, nor the accompanying drawings, act to define or otherwise limit the inventive subject matter to which the inventors and/or the inventors&#39; assignee(s) claim exclusive rights; that is the exclusive role of the claims which follow. Accordingly, this description should not be read to limit the scope of the claimed subject matter, nor imply any preference of the described embodiments over any other embodiment, unless such a limitation or preference is explicitly identified herein. For example, although the illustrative embodiments focus on a EUV transparent support mesh for use with a pellicle in EUV lithography, the inventive support mesh defined by the claims are intentionally not so limited. 
       FIG. 1A  illustrates a cross-section of a pellicle  300  suitable for use in EUV lithography and incorporating an ultrathin membrane  304  and aspects of the present supportive mesh  308 . Advantageously, a pellicle  300  manufactured in accordance with aspects of the methods described below will be a monolithic structure, with no adhesive necessary at the junction  312  between membrane  304  and supportive mesh  308 . The membrane  304  advantageously has a thickness of less than or equal to twenty five nanometers (25 nm) and composed of materials that absorb low levels of EUV radiation. For example, a composite membrane composed of ZrB 2  and Zr is suitable. 
     The supportive mesh  308  is advantageously made of a relatively strong material and low levels of EUV absorption. For example, a zirconium or boron mesh is suitable. Preferably, the pellicle  300  will have a composite EUV transmission rate of at least ninety percent (90%). 
     In a preferred embodiment of the present pellicle  300 , the thickness t M  of the Zr/ZrB 2  composite membrane is preferably twenty nanometers (20 nm). The bars of the supportive mesh  308  preferably have a width W B  between one half (½) and two (2) microns, a thickness is of fifty (50) to two hundred (200) nanometers, and a pitch of twenty five (100) to three hundred (300) micrometers. A boron mesh manufactured in accordance with the present methods and having such preferred dimensions has been found to have an average EUV transparency of ninety six percent (96%) to ninety nine point seven percent (99.7%). 
     Referring to  FIG. 1B , the supportive support bars  308  may advantageously be arranged to form a supportive mesh  308  on a surface of the membrane  304 . The supportive mesh may advantageously be arranged in a hexagonal pattern, or any other pattern that provides the necessary structural support to the membrane  304 . In addition to providing structural support to the membrane  304  and rigidity to the pellicle  300 , the supportive mesh further may advantageously act to arrest cracks which may form in the membrane  304 . To successfully arrest a crack, the force propagating the crack tip must be less than the tensile strength of the bars of the supportive mesh. Via Griffith&#39;s crack criterion, a bar should be sufficient to arrest propagation of a crack in the membrane if the ratio of the bar thickness to membrane thickness is at least 3: 
                 t   B       t   M       &gt;     C       σ   Y     ⁢     a               
Where t B  is the mesh bar thickness, t M  is the membrane thickness, C is the fracture toughness of the membrane, σ Y  is the yield strength of the bar, and a is the pitch of the bar.
 
       FIGS. 2A through 2I  depict aspects of a method of manufacturing a monolithic, EUV transparent membrane and supporting mesh, such as, but not limited to, the pellicle embodiments described above.  FIG. 2B  shows a partial cross-section of a substrate  404 , such as a silicon wafer, with a protection layer  408  of a release agent applied thereto. Next, as is shown in  FIG. 2C , a uniform, ultrathin layer of EUV transparent membrane material  412  is deposited onto the protection layer  408 . The layer of membrane material may preferably have a thickness ranging from less than one nanometer up to thirty (30) nanometers, depending on the characteristics, such as material strength and EUV transparency, of the material being used. In at least one preferred embodiment, the thickness of the membrane material is less than twenty five nanometers. 
     In  FIG. 2D , a layer of photoresist  416  is deposited over the membrane layer  412 .  FIG. 2E  depicts a lithographic patterning step, wherein the photoresist layer  416  is exposed to an ultraviolet light source  417 . Interposed between the photoresist layer  416  and the ultraviolet light source  417  is a mask  418  having a two-dimensional pattern corresponding to the negative of the desired shape of the supporting mesh, such as the hexagonal pattern depicted in  FIG. 1B . As shown in  FIG. 2F , the ultraviolet light causes the areas of the photoresist layer  416  which are exposed to it to dissolve, leaving gaps  419  in the photoresist layer  416  corresponding do the desired shape of the supportive mesh. 
     Referring to  FIG. 2G , the gaps  419  in the photoresist layer  416  are then filled with a layer of material suitable for use as the supportive mesh, such as zirconium, boron, zirconium diboride (“ZrB 2 ”), carbon, silicon, niobium, and molybdenum. The layer of mesh material may preferably have a thickness ranging from less than one nanometer up to thirty (30) micrometers, depending on the characteristics, such as material strength and EUV transparency, of the material being used. In at least one preferred embodiment, the thickness of the mesh layer is between one hundred (100) and two hundred (200) nanometers. Due to the nature of the materials used and the lithographic process, the supporting mesh material bonds with the material of the membrane layer, forming a monolithic structure composed of the membrane material and the supporting mesh material. 
     Next, as is shown in  FIG. 2G , the remaining photo resist is dissolved, leaving the monolithic combination of the supportive mesh  420  and the membrane  412 . protection layer  408  are then removed allowing the monolithic structure to be removed from the substrate  404 . 
     Exemplary embodiments of the present methods and systems have been described in detail above and in the accompanying figures for illustrative purposes. However, the scope of the present methods and systems are defined by the claims below and is not limited to the embodiments described above or depicted in the figures. Embodiments differing from those described and shown herein, but still within the scope of the defined methods and systems are envisioned by the inventors and will be apparent to persons having ordinary skill in the relevant art in view of this specification as a whole. The inventors intend for the defined methods and systems to be practiced other than as explicitly described herein. Accordingly, the defined methods and systems encompass all modifications and equivalents of the subject matter as permitted by applicable law.