Abstract:
In the invention a pellicle mounting structural principle is provided whereby a membrane for protection of an X-ray mask is interchangeably positioned with proper spacing between the X-ray mask and the resist on the wafer in which the pattern produced by the X-ray exposure is to be formed. The mounting principle employs a combined assembly of, a membrane and spacer member subassembly together with a means for seectably separable retention to the supporting structural portion of the mask The principle accommodates membrane materials that may not be flexible and provides an ability to remove the membrane for cleaning or replacement and to removal and reassembly with ease in reestablishing the spacing with respect to the mask. The means for the selectably separable retention to the supporting structural portion of the mask involves the use of springs and elastomers, securing to the sides of the supporting structural mask ring and the bonding of the spacer member directly to the mask.

Description:
CROSS REFERENCE 
     Ser. No. 09/335,536 filed assigned to the assignee of this application and is part of an ongoing effort in this technology and which is incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The invention relates to pattern replication using x-rays and in particular to an X-ray mask mounting device called a pellicle, which serves to provide a spacing function and to protect the x-ray mask from contamination in semiconductor manufacturing. 
     BACKGROUND OF THE INVENTION AND RELATION TO THE PRIOR ART 
     The manufacturing of semiconductor integrated circuit devices is dependent upon the accurate replication of generated patterns onto the surface of a device substrate. This is usually accomplished by the production of a computer generated pattern into a chromium layer on a quartz substrate, and the pattern is then transferred via optical lithography. The replication operation is effected using a variety of processes; such as subtractive, for example etching; additive, for example deposition; and, by using such material modification techniques as oxidation, and ion implantation. Heretofore in the art, optical lithography, which is a projection printing technique has been employed in the replication process. In optical lithography the mask is located some distance from the wafer to be exposed and a four or five times reduction between the mask image and the wafer image can be involved, which simplifies both the lithography and mask production and in turn provides tolerance of defects. 
     As the art is progressing, the desire for greater density, is making the use of X-ray replication directly on the wafer very attractive. The considerations in using X-ray replication however are quite formidable. The X-ray technique involves proximity replication so that the X-ray mask images are made the same size as the final images on the wafer. Because of the one-to-one relationship of the image on an X-ray mask and the image formed on the wafer, any position errors in building the X-ray mask are replicated one for one onto the wafer. Thus, the position accuracy requirements for the fabrication of X-ray masks are very difficult to achieve and, as a result, X-ray masks are expensive to fabricate. In use, particles unavoidably settle on masks, but the ability, previously available in optical lithography, of keeping settled particles outside of the depth of focus window in the replication so that the particle is highly de-focused and does not print, is no longer available in the X-ray proximity replication. In some situations a particle that settled on a mask can absorb X-ray photons and must be removed. A major source of a settling particle problem comes from the effect of the X-ray energy on the resist material that form the desired patterns. When the x-ray resist is irradiated, organic material is released from the resist Because X-ray lithography is a proximity printing process, this material will travel the short distance between the mask and the wafer and then land on the x-ray mask. In particular, this material is prone to bonding to the x-ray mask absorber pattern. In a short time, this organic material begins to attenuate the x-rays resulting in longer exposure times, dimensional control problems, and pattern defects. This accumulation of contamination would ordinarily necessitate cleaning of the x-ray mask. However, due to the expense and fragility of the x-ray masks, and the risk involved with cleaning them, this is not considered to be a reasonable solution. 
     It is therefore important to protect the relatively delicate and not easily cleaned X-ray masks from contaminants and mechanical damage due to scratches and the like without introducing mechanical stresses that may affect the placement accuracy. 
     There has been some activity in the art directed to this problem in U.S. Pat. No. 5,793,836 in which a protective structure is described which provides a membrane that would lie between the mask and the wafer to assist in protecting the mask from contamination. 
     As progress with membranes advances a need is being encountered for less fragile and easier to mount structures. 
     SUMMARY OF THE INVENTION 
     In the invention a pellicle mounting structural principle is provided whereby a membrane for protection of an X-ray mask is interchangeably positioned with proper spacing between the X-ray mask and the resist on the wafer in which the pattern produced by the X-ray exposure is to be formed. The mounting principle employs a combined, membrane and rigidity and spacer member, together with a means for selectably separable retention to the supporting structural portion of the mask. The principle accommodates membrane materials that may not be flexible and provides an ability to remove the membrane for cleaning or replacement and to do it with ease in reestablishing the spacing with respect to the mask The means for the selectably separable retention to the supporting structural portion of the mask involves the use of springs and elastomers, securing to the sides of the supporting structural mask ring and the bonding of the spacer member directly to the mask 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an expanded cross sectional diagram of the prior art x-ray lithography showing the configuration of the x-ray mask with respect to the device wafer. 
     FIG. 2 is a cross sectional depiction of the pellicle mounting principle of the invention showing the combined membrane and rigidity and spacer member together with a means for selectably separable retention to the rigidity imparting portion of the mask 
     FIG. 3 is an exploded view of the x-ray pellicle mounting principle of the invention as it would be used in conjunction with an x-ray mask and wafer. 
     FIG. 4 is a cross sectional view that shows a spring loaded embodiment of the invention. 
     FIG. 5 is a detailed view of one spring loaded portion of FIG. 4 
     FIG. 6 is a cross sectional view that shows an elastomer retention embodiment of the invention. 
     FIG. 7 is a detailed view of one one elastomer retention portion of FIG.  6 . 
     FIG. 8 is a cross sectional view in which a first side of a single retention ring is first glued to the pellicle using an elastic adhesive and the second side of the retention ring is attached to the support ring of the x-ray mask. 
     FIG. 9 is a detailed view of one one retention portion of FIG.  8 . 
     FIGS. 10-13 shows plan, elevational, perspective and cross sectional views of an embodiment of the invention in which a single ring has slots cut in it to produce tabs which frictionally secure the pellicle to the edge of the X -ray mask supporting ring. 
     FIG. 14 shows an embodiment of the invention in which a ring with an outside thread is glued to the outer diameter of the supporting ring of the x-ray mask and a pellicle ring is threaded on the interior diameter such that the pellicle structure can be screwed onto the x-ray mask. 
     FIG. 15 is a detailed view of one threaded retention portion of FIG.  14 . 
     FIG. 16 shows an embodiment of the invention in which the spacer member of the pellicle is bonded directly to the surface of the x-ray mask using an electrostatic charge 
    
    
     DESCRIPTION OF THE INVENTION 
     A pellicle mounting approach is provided in which a membrane, that protects the X-ray mask from contaminating particles that principally come out of the resist on the wafer and settle on the mask, is mounted so that the membrane not only keeps contamination off the mask, the mounting pellicle also serves a membrane spacing function rendering the pellicle readily and accurately removable for cleaning or replacement 
     Referring to FIG. 1 there is shown an exploded cross sectional view of a typical prior art x-ray mask  1  that is positioned in proximity printing position separated by a relatively narrow gap  2  of the order of about 5 to 40 micrometers in relation to an X-ray resist  3  that is on an integrated circuit wafer  4 . The mask  1  is made up of a thinned region  5  about 2 micrometers thick and about 1 inch square that is supported at the periphery by a bulk support portion  6  of for example silicon, which in turn is mounted on and supported by a structural support ring  7  of a material such as pyrex(TM) that is about 4 inches in diameter. On the face  8  of the thin region  5  of the mask  1  there is an absorber pattern  9  of X-ray opaque material of for example Au, W, or TaSi. The absorber pattern  9  is transferred via x-rays  10  across the gap  2  onto a x-ray resist  3 , which is on a device wafer  4 . A major problem in the prior art of the type depicted in FIG. 1 is that contaminating particles usually of organic material driven out of the resist  3  by the X-rays tend to settle on the mask and cause a need for cleaning. Any cleaning however must be done with a realization that the pattern, being a one for one replica, will require great care because it is delicate and will be expensive to reproduce if damaged. Referring to FIG. 2 there is shown a cross sectional depiction of the basic structural aspects of the pellicle mounting in accordance with the invention. In the invention there is provided a membrane  11  of the order of about 1 micrometer thick of a material with minimal X-ray attenuation and which does not distort the X-ray pattern and can serve to protect the absorber pattern  9  from wear and contamination. The membrane  11  is supported by and given rigidity through, a spacer member  12  that has a thickness slightly greater than the thickness of the absorber pattern  9 , as shown in FIG. 1, extending from the surface  8  of the mask  1 , which is about  2  micrometers. The pellicle mounting of FIG. 2 is positioned in the gap labelled  2  in FIG. 1 between the absorber pattern  9  and the resist  3  on the wafer  4  being patterned. In the pellicle mounting structure of the invention as shown in FIG. 2, the membrane  11  is perfectly flat in intimate contact with the spacer  12  and is free of any bending requirement, so that it can be of a wider range of materials including for example inorganic material such as a member of the group of Si, Si 3 N 4 , SiC, or Diamond. The spacer member  12  can be of any material that can impart the desired rigidity and can withstand the processing temperatures likely to be encountered. The spacer member  12  should be in a shape similar to a washer that does not extend into an open region  13  through which the energy through the absorber pattern  9  of the X-ray mask is to pass. The washer shaped spacer member  12  with the membrane  11  over one surface and across the open region  13  is made by standard procedures in the art such as chemical vapor deposition of the membrane  11  on a wafer with a boron diffused layer of the desired thickness in the shape of a washer followed, by removal of the region  13  by silicon wet etching. The membrane  11  and spacer member  12  are somewhat fragile and are provided with retention means for handling and that attach the combination to the mask, in position within the gap  2  distance, and with the absorber pattern  9  of the mask surrounded by the spacer member  12 . The retention means can be a peripheral attachment member or merely the attachment of the spacer  12  to the surface  8  of the mask. The membrane  11  and spacer  12  combination basic structure of the invention as illustrated in FIG. 2, make it possible to use inorganic and high processing temperature materials that are amenable to precision processing. The use of boron doped silicon for the spacer  12  and for the membrane  11  has the advantage of being compatible with high temperature processes up to 1100° C., and permits the use of well understood and readily available silicon etchants. Further, a pellicle mounting element of such a material has low tensile stress. 
     Where the retention means is a peripheral ring as shown dotted in FIG. 2 as element  14 , the material silicon as the material for the ring will add ridgidity during processing and mounting as well as preventing any defects to the planarity of the membrane surface. 
     FIG. 3 is an expanded, cross sectional, separated as shown dotted, view of the x-ray pellicle mounting principle of the invention wherein the same reference numerals as in previous figures are used where appropriate, and as it would be used in conjunction with a standard x-ray mask and wafer as shown in FIG.  1 . When the expanded view is in use, the resist  3  on the wafer  4  would be in close proximity and separated by a gap from the membrane  11 ; one surface of the washer shaped spacer  12  would be in contact with the face  8  of the mask  1 ; and the absorption pattern  9  of the mask  1  would be in the opening  13  in the spacer surrounded by the spacer  12 . Thus, the absorption pattern  9  is protected on the sides and over the face near the resist  3  it is protected from contamination by the membrane  11 . The thickness of the spacer  12  positions the membrane  11  in the gap  2 , proximate to but with a clearance of about 1 micrometer from the absorption pattern  9 . 
     The retention means for attaching the pellicle mounting in one embodiment can be a peripheral ring  14  as shown dotted in FIG. 2 that is bonded or glued to the portion of the spacer member  12  beyond the mask  1  and to mask structural support ring  7  at a shoulder  15 . Thus the retention means, in the ring  14  embodiment retains the pellicle assembly on the mask  1  with the surface of the spacer  12  in contact on one side with the face  8  of the mask  1  and with the membrane  11  in contact with the other surface of the spacer  12 . The washer shaped spacer  12  surrounds the absorption pattern  9  of the mask  1  and establishes the position of the membrane  11  in the relatively narrow about 5 to 40 micrometer gap  2  with the membrane  11  covering, with a small clearance, the absorption pattern  9 . The ring  14  embodiment is attached on one edge to the support ring  7  and on the other edge to the spacer  12  when the pellicle is assembled on the mask  1  with the upper surface of the washer shaped spacer  12  in contact with the face  8  of the mask  1 . 
     In another embodiment of the retention means requirement of the pellicle mounting of the invention, a spring and guiding rod between two superimposed ring portions may be used. In this type of embodiment, the overall mask and pellicle structure is shown in cross section in FIG. 4, using the same reference numerals as in previous figures where appropriate, and a larger detail of one of the spring and guiding rod assembles is shown in FIG,  5 . There are as many spring and guiding rod assemblies around the structural support ring  7  as are needed for maintaining planarity of the parts. Referring to FIGS. 4 and 5 a spring and superimposed ring assembly  16 , provide the function of a retention means in securing the x-ray pellicle assembly of the spacer  12  and the membrane to the x-ray mask  1 . Superimposed rings  17  and  18  are connected by a spring  19  that will be under tension when the ring  17  is retained to the ring  7  by a screw  20  with threads  21  in the ring  17  and where the ring  18  is bonded such as through an adhesive like epoxy to the spacer  12  at location  22 . A guiding rod  23  is attached to ring  18  and extends to the tip of screw  20  thereby giving the assembly lateral rigidity. When assembled with the screw  20  drawn down tight to the structural support ring  7 , the spring  19  is stretched slightly to provide a small force on the spacer  12  and membrane  11  combination. The spring  19  force must be small enough so as not to damage the membrane  11 . 
     In still another embodiment of the retention means requirement of the pellicle mounting of the invention, an elastomer  25  applied across two superimposed ring portions may be used. In this type of embodiment, the overall mask and pellicle structure is shown in cross section in FIG. 6, using the same reference numerals as in previous figures where appropriate, and a larger detail of one of the elastomer and superimposed ring assembles is shown in Fig,  7 . There are as many elastomer and superimposed ring assemblies around the structural support ring  7  as are needed for maintaining planarity of the parts. 
     Referring to FIGS. 6 and 7 an elastomer  25  and superimposed ring  26  and  27  assembly  28 , in which the elastomer is bonded across the intersection  29  between the superimposed rings  26  and  27 , provide the function of a retention means in securing the x-ray pellicle assembly of the spacer  12  and the membrane  11  to the x-ray mask  1 . Of the superimposed rings  26  and  27 , the ring  26  is retained to the ring  7  by a screw  30  with threads  31  in the ring  26  and the ring  27  is bonded such as through an adhesive like epoxy to the spacer  12  at location  32 . When assembled with the screw  30  drawn down tight to the structural support ring  7 , the elastomer  25  is stretched slightly to provide a small force on the spacer  12  and membrane  11  combination. The elastomer force  25  must be small enough so as not to damage the membrane  11 . 
     In still another embodiment of the retention means requirement of the pellicle mounting of the invention, an elastic adhesive and a ring with a dimension so that the elastic adhesive is stretched when the pellicle is mounted is used. In this type of embodiment, the overall mask and pellicle structure is shown in cross section in FIG. 8, using the same reference numerals as in previous figures where appropriate, and a larger detail of one of the ring and elastic adhesive assembles is shown in Fig,  9 . 
     Referring to FIGS. 8 and 9 a ring  33  and an elastic adhesive  34  assembly  35 , is positioned between the support ring  7  and the spacer  12  when the pellicle is mounted on the mask  1  and provide the function of a retention means in securing assembly of the spacer  12  and the membrane  11  to the x-ray mask  1 . The height dimension of the ring  33  is less than the distance between the support ring  7  and the spacer  12  so that an elastic adhesive  34  at the location  36  is stretched and provides a force joining the spacer  12  to the support ring  7  when the parts are assembled. The ring  33  is attached to the support ring  7  through a screw  37  that places the elastic adhesive under tension when the screw  37  is tightened which in turn provides a small force on the spacer  12  and membrane  11  combination. The elastic adhesive  34  force must be small enough so as not to damage the membrane  11 . 
     Referring next to FIGS. 10-13 which show plan, elevational, perspective and cross sectional views of a friction attachment embodiment of the retention means requirement of the pellicle mounting of the invention, in which a single attachment ring  40  has tabs  41  bent inward from portions formed by slots  42 . The tabs  4  operate to grip the edge of the X ray mask supporting ring.  7  and thereby retain the spacer  12  and membrane  11  combination in position on the X-ray mask. FIG. 10 is a top view in which the tabs  41  have been bent toward the supporting ring  7  which the attachment ring  40  surrounds. For this embodiment, the attachment ring  40  would likely be made of a material with a high spring property such as beryllium copper. FIG. 11 is a side view illustrating the tab  41  producing slots  42  in which the tab  41  portions are formed by a slot  42  that is transverse to the edges for about half the distance between the edges then parallel to the edges for a distance about twice the distance between the edges. FIG. 12 is a perspective view of the ring  40  with the tabs  41  bent inward. The attachment friction increases with the number of tabs and the distance they are bent inward. Referring to FIG. 13, to implement this embodiment, portion of the ring  40  that is the side opposite the tabs  41  would be glued to the spacer  12 . The tabs  41  would be stretched outward slightly to allow the ring  40  to fit over the portion  6  of the x-ray mask. When the tabs  41  are released they will grip the sides of the portion  6  and the structural support ring  7  of the x-ray mask below the shoulder  15 , securing the spacer  12  and membrane  11  combination in place. 
     It will be apparent to one skilled in the art that the principles of the invention apply as well to appropriately mounted masks with shapes other than circular such as square. 
     In still another embodiment of the retention means requirement of the pellicle mounting of the invention, an arrangement for the use of screw threads is provided. In this type of embodiment, the overall mask and pellicle structure is shown in cross section in FIG. 14, using the same reference numerals as in previous figures where appropriate, and a larger detail of the threaded intersection assembly is shown in FIG.  15 . 
     Referring to FIGS. 14 and 15 a ring  45  threaded on the outer diameter is glued to the outer diameter of the structural support ring  7  of the X-ray mask. A pellicle retention ring  46  that is threaded on the inner diameter with a matching thread to that of ring  45 , is glued or otherwise attached such as through the use of epoxy, at location  47  to the spacer  12 . 
     In mounting the combination of the membrane  11  and the spacer  12  to the X-ray mask structural support ring  7  using the retention means made up of threaded rings  45  and  46  the ring  45  is screwed onto ring  45  until the mask portion  6  contacts the surface of the spacer  12  at location  48 . 
     In FIG. 16 an embodiment is depicted in which the retention means for holding the spacer  12  and membrane  11  combination on the X-ray mask is provided by a charge accumulation across the interface between the spacer  12  and the X-ray support ring  7 . In the embodiment of FIG. 16 the spacer is made of bulk silicon and is formed directly on the surface of the X-ray mask. A direct current voltage source  55  is connected across the interface and with a dielectric film at the interface a capacitive charge accumulation occurs producing an electrical attraction between the parts that holds the pellicle in place even after the voltage source  55  is disconnected. 
     What has been described is a pellicle mounting structural principle whereby a membrane for protection of an X-ray mask is removably positioned between the absorber pattern on the X-ray mask and the resist on the wafer.