Abstract:
A new method is provided for the creation of a seal ring or fuse ring over the surface of a Phase Shift Mask. A seal ring pattern is created over the surface of a phase shift mask through a layer of phase shift material and a layer of opaque material. The seal ring is surrounded by a layer of opaque material by etching the layer of opaque material.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method for using a seal ring for a Phase Shift Mask in such a manner that no side lobe ringing problems occur. 
     (2) Description of the Prior Art 
     A photolithographic mask provides the ability to selectively expose the surface of a layer of light sensitive material for the creation of semiconductor patterns therein. The photolithographic mask contains a pattern of device features that are transposed from the mask to underlying layers of photosensitive semiconductor material such as a layer of photoresist. A standard photo mask contains a transparent substrate, typically made of quartz. A patterned layer of opaque material is created over a surface of the transparent substrate. Chromium is typically used for the opaque material, deposited to a thickness of about 1,000 Angstrom. Alternate opaque materials for the creation of the patterned layer over the surface of a photolithographic mask are nickel and aluminum. 
     The principle of phase shifting of the light as the light passes through the mask is widely applied in the creation of photolithographic masks. Phase shifting masks are used to create device features of sub-micron dimensions. It is well known in the art that adjacent light beams, which are in extreme close proximity to each other while the light beams are used to create sub-micron devices features, mutually influence each other, having a detrimental effect on the definition of the exposed patter. The phase shift mask counteracts this mutual influence that closely spaced light beams have on each other. As a further advance, alternate phase shifting masks are used, where the phase shifting characteristic of the phase shifting mask is alternately counteracted in the light as the light passes through the photo mask. 
     In the creation of photolithographic mask difficulties are encountered by the transition or boundaries between light transmitting and light blocking surface areas of the mask. Light that passes close to these boundaries is diffracted, resulting in loss of sharpness of definition of the exposed and therefrom created pattern. Ideally, the exposing light passes through the transparent surface of the mask in an equal amount of energy, even where the boundary of the transparent surface of the mask is approached. The opposite is ideally true of the opaque surface area of the mask: the light should ideally be completely blocked from passing through the surface area of the mask that is covered with opaque material, also independent of the boundary of the opaque surface area being approached. In actual implementation however light will be diffracted due to the existence of the boundary between the transparent and the opaque surface areas of the mask, a defraction that reduces definition of the created pattern. Light defraction that occurs at boundaries between transparent and opaque surface areas of a photolithographic mask is generally referred to as side lobe ringing of the transmitted light. 
     To compensate for effects of light defraction, methods have been provided for the compensation thereof by for instance, U.S. Pat. No. 5,795,682 (Garza), the creation of guard rings which provide compensating light transmissive regions that are located such that side lobe ringing is most effectively counteracted. These compensating light transmissive regions do not contribute to the definition of a pattern in the exposed surface. The effectiveness of the compensating transmissive regions is assured due to the fact that the radiation that causes the side lobe ringing is about 180 degrees out of phase with the radiation that passes through the transmissive regions. 
     The invention does not address the creation of guard rings as these guard rings have been provided by U.S. Pat. No. 5,795,682 (Garza). In the creation of a PSM, a seal ring image is provided over the surface of the mask. This seal ring image is projected onto the exposed surface, surrounding the active surface area of the exposed chip for the protection of the chip during the process of sawing or singulating the chip. This seal ring provided in or over the surface of the chip is typically a relatively wide line of metal, comprising overlaying layers of metal-1 through metal-n, that are interconnected with vias. This seal ring serves as a barrier and provides protection against mostly moisture penetration into the chip. 
     The instant invention addresses the creation of the seal ring image, also referred to as a fuse guard ring, over the surface of the photolithographic mask. 
     U.S. Pat. No. 5,795,682 (Garza) shows a guard ring to compensate for sidelobe ringing in an APSM. 
     U.S. Pat. No. 6,355,503 B2 (Schroeder) shows square contact holes utilizing a sidelobe formation. 
     U.S. Pat. No. 6,291,113 B1 (Spence) shows a side lobe suppressing PSM. 
     U.S. Pat. No. 6,210,841 B1 (Lin et al.) is a related APSM patent. 
     SUMMARY OF THE INVENTION 
     A principle objective of the invention is to provide a Phase Shift Mask wherein sidelobe occurrence associated with the seal ring or fuse guard ring are reduced. 
     In accordance with the objectives of the invention a new method is provided for the creation of a seal ring or fuse ring over the surface of a Phase Shift Mask. A seal ring pattern is created over the surface of a phase shift mask through a layer of phase shift material and a layer of opaque material. The seal ring is surrounded by a layer of opaque material by etching the layer of opaque material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a top view of a conventional photolithographic exposure mask, highlighting the seal ring thereof. 
         FIG. 2  shows a cross section of the seal ring that is created around a perimeter of a semiconductor chip. 
         FIGS. 3 through 6  address the invention, as follows: 
         FIG. 3  shows the cross section of a quartz surface over which have been deposited a layer of phase shifting material and a layer of opaque material, a first photoresist mask has been created. 
         FIG. 4  shows a cross section of the quartz surface after the layers of opaque material and phase shift material have been etched in accordance with the first photoresist mask, the first photoresist mask has been removed. 
         FIG. 5  shows the creation of a second photoresist mask. 
         FIG. 6  shows a cross section after the layer of opaque material ahs been etched in accordance with the second photoresist mask. 
         FIGS. 7 and 8  show the side lobe suppression as achieved by the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For half-tone shift masks the shift material is typically MoSiON. The standard mask comprises a transparent substrate on the surface of which a patterned layer of opaque material has been created. Typically used for the opaque material is chromium that has been deposited over the quartz substrate to a thickness of about 1,000 Angstrom. Alternate opaque materials for the creation of the patterned layer on the surface of a photolithographic mask are nickel and aluminum while for more sophisticated masks such as halftone phase shift masks MoSiON is used as the phase shift material. For the substrate most typically is used quartz whereby however glass and sapphire can also be used for this purpose. 
     The cross section that is shown in  FIG. 1  is a top view of a photolithographic exposure mask  10  wherein specifically are highlighted the seal ring exposure area  12  that surrounds the surface area of the mask in which conventional patterns are created. 
     It has previously been indicated that created semiconductor devices are protected against outside influences by a seal or fuse guard ring, the cross section of such a seal ring is shown in  FIG. 2  wherein are highlighted the seal ring as comprising overlying layers  17  of seal ring metal that are interconnected by overlying and therewith aligned vias  18 . It is the creation of this seal ring or fuse guard ring that is addressed by the invention. 
     The invention is now described in detail using  FIG. 3 through 6  for this purpose. 
     Shows in the cross section of  FIG. 3  is a transparent surface  20 , typically comprising quartz, over the surface of which have been deposited a layer  22  of phase shift material such as MoSiON and a layer  24  of opaque material, such as chrome. A first photoresist mask  26  has been created over the surface of the layer  24  of chrome, using conventional methods of photolithographic exposure and development. Resist mask  26  preferably comprises E-beam resist and defines the seal ring or fuse guard pattern. The pattern  25  that is created through the first photoresist mask is the pattern of the seal ring  12 , FIG.  1 . The cross-sections that are shown in  FIGS. 3-6  are therefore cross-section that are limited to the perimeter of an exposure mask. 
     The layers  24  of chrome and  22  of MoSiON are etched in accordance with the first photoresist pattern  26 , the results of which are shown in the cross section of FIG.  4 . The first photoresist pattern  26  has been removed from the surface of layer  24  using conventional methods of resist ashing followed by a thorough surface clean. 
     Typical processing conditions for the application of an isotropic etch, applied to the surface of layer  221  of MoSiON, are using a pressure of about 150 mTorr, using CF 4  and O 2  as etchants at a flow ratio of 100:10 (100 sccm CF 4  and 10 sccm O 2 ) and an applied power of about 200 watts. 
     A second photoresist pattern  28  is now created, as shown in the cross section of  FIG. 5 , whereby the second resist pattern  28  overlies the surface of layer  24  of chrome over a distance  27  that extends to both sides of the openings  25  created through layers  24  and  22 . 
     Using the second photoresist pattern  28  as a mask, the layer  24  of chrome is etched, leaving in place a patterned and etched layer of chrome that surrounds the seal ring or guard ring opening  25 . It is this patterned and etched layer  28  of chrome that is shown in cross section in  FIG. 6  which serves as sidelobe inhibitor during the process of creating the seal ring or the guard ring of the semiconductor device. 
     This latter effect is further highlighted in  FIGS. 7 and 8 , whereby the intensity of the light that is transmitted through the exposure mask is plotted along a vertical axis as a function of distance. At the center of opening  25 , the exposure is of a maximum value from which the exposure intensity decreases as the perimeter of opening  25  is approached. This is shown in the graph of  FIG. 7  using curve “a”. Curve “b” in the same graph shows the effect of the layer  24  of chrome since this layer comprises an opaque material and therefore affects a reduction in light transmission. 
     The results that follow from the modified intensity pattern of the light that is transmitted through the exposure mask are shown in the cross sections of an exposed layer  26  of photoresist. The profile “a” of the exposed layer of photoresist corresponds to curve “a” in FIG.  7  and therefore shown a relatively severe dip in the exposure density, resulting in a photoresist removal profile that follows profile “a” of FIG.  8 . 
     The profile “b” of the exposed layer of photoresist corresponds to curve “b” in FIG.  7  and therefore shows a sharply reduced severe dip in the exposure density, resulting in a photoresist removal profile that follows profile “b” of FIG.  8 . 
     By comparing the two photoresist removal profiles “a” and “b”, it is clear that the patterned and etched layer  24 ,  FIG. 6 , of chrome results in a photoresist removal profile that more closely resembles an ideal profile, which ideally should resemble a square or rectangle. 
     To summarize the invention: 
     1. The invention provides a “forbidden zone”, the patterned and etched layer  24 ,  FIG. 6 , of chrome overlaying the layer of phase shift material, this “forbidden zone” provides sidelobe prevention in creating a seal ring or guard ring over the surface of a semiconductor chip 
     2. The patterned and etched layer  24 ,  FIG. 6 , of chrome remains in place adjacent to the profile of the seal ring and the guard ring 
     3. The invention requires two photoresist masks for the creation of the sidelobe prevention 
     4. The dimension of the sidelobe prevention, that is dimension  27 ,  FIG. 5 , is preferred to be in the range between 0.1 and 200 μm. 
     The creation of the vias  18 ,  FIG. 2 , that are conventionally part of a seal ring, is not part of the invention. 
     Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.