Abstract:
The present invention generally relates to optical lithography and more particularly relates to the fabrication of transparent or semitransparent phase shifting masks used in the manufacture of semiconductor devices. In particular, the present invention utilizes a light absorbing film in a conventional aaPSMs to balance the intensity of light through each opening of the photomask. The aaPSM of the present invention is used to make semiconductor devices or integrated circuits.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to optical lithography and more particularly relates to improved alternating aperture phase shift masks (“aaPSMs”) and methods of making the same. 
     BACKGROUND OF THE INVENTION 
     Photomasks are high precision plates containing microscopic images of electronic circuits. Photomasks are typically made from flat pieces of material that are substantially transparent, such as quartz or glass, with an opaque layer, such as chrome, on one side. Etched in the opaque layer (e.g., chrome) of the mask is a pattern corresponding to a portion of an electronic circuit design. A variety of different photomasks, including for example, aaPSMs, embedded attenuated phase shift masks and binary photomasks (e.g., chrome-on-glass), are used in semiconductor processing to transfer these patterns onto a semiconductor wafer or other type of wafer. 
     As shown in  FIGS. 1   a  and  1   b , to create an image on a semiconductor wafer  20 , a photomask  9  is interposed between the semiconductor wafer  20  (which includes a layer of photosensitive material) and an optical system  22 . Energy generated by an energy source  23 , commonly referred to as a Stepper, is inhibited from passing through opaque areas of the photomask  9 . Likewise, energy from the Stepper passes through the substantially transparent portions of the photomask  9 , thereby projecting a diffraction limited, latent image of the pattern on the photomask onto the semiconductor wafer  20 . In this regard, the energy generated by the Stepper causes a reaction in the photosensitive material on the semiconductor wafer such that the solubility of the photosensitive material is changed in areas exposed to the energy. Thereafter, the photosensitive material (either exposed or unexposed) is removed from the semiconductor wafer  20 , depending upon the type of photolithographic process being used. For example, where a positive photolithographic process is implemented, the exposed photosensitive material becomes soluble and is removed. By contrast, where a negative photolithographic process is used, the exposed photosensitive material becomes insoluble and the unexposed, soluble photosensitive material is removed. After the appropriate photosensitive material is removed, a pattern corresponding to the photomask  9  appears on the semiconductor wafer  20 . Thereafter, the semiconductor wafer  20  can be used for deposition, etching, and/or ion implantation processes in any combination to form an integrated circuit. 
     As circuit designs have become increasingly complex, semiconductor manufacturing processes have become more sophisticated to meet the requirements of these complexities. In this regard, devices on semiconductor wafers have continued to shrink while circuit densities have continued to increase. This has resulted in an increased use of devices packed with smaller feature sizes, narrower widths and decreased spacing between interconnecting lines. For photolithographic processes, resolution and depth of focus (DoF) are important parameters in obtaining high fidelity of pattern reproduction from a photomask to a wafer. However, as feature sizes continue to decrease, the devices&#39; sensitivity to the varying exposure tool wavelengths (e.g., 248 nm, 193 nm, 157 nm, 13 nm, etc.) used to write images on a semiconductor wafer has increased, thereby making it more and more difficult to write to an accurate image on the semiconductor wafer. In this regard, as feature sizes continue to decrease, light diffraction effects in the photomask are exacerbated, thereby increasing the likelihood that defects will manifest in a pattern written on a semiconductor wafer. Accordingly, it has become necessary to develop new methods to minimize the problems associated with these smaller feature sizes. 
     One known method for increasing resolution in smaller feature sizes involves the use of shorter exposure wavelengths (e.g., 248 nm, 193 nm, 157 nm, 13 nm, etc.). Shorter exposure wavelengths, however, typically result in a shallower DoF in conventional binary chrome-on-glass (COG) photomasks having smaller feature sizes. In this regard, when the feature size is smaller than the exposure tool wavelength, binary COG photomasks become diffraction limited, thereby making it difficult, if not impossible, to write an accurate image on the semiconductor wafer. Accordingly, phase shifting masks (“PSMs”) have been used to overcome this problem. In this regard, PSMs are known to have properties which permit high resolution while maintaining a sufficient DoF. More particularly, a PSM reduces the diffraction limitation ordinarily associated with a binary COG mask by passing light through substantially transparent areas (e.g., glass or quartz) which have either different thickness and/or different refractive indices than an ordinary binary COG mask. As a result, destructive interference is created in regions on the target semiconductor wafer that are designed to see no exposure. Thus, by reducing the impact of diffraction through phase shifting, the overall printability of an image is vastly improved such that the minimum width of a pattern resolved by using a PSM is approximately half the width of a pattern resolved in using an ordinary binary COG mask. 
     Various types of PSMs have been developed and are known in the art, including aaPSMs.  FIGS. 2   a-b  illustrate an example of a conventional aaPSM  10 . An aaPSM is typically comprised of a layer of opaque material and a substantially transparent substrate which is etched on one side of the opaque features, while not etched on the other side (i.e., etching of the transparent substrate occurs in alternating locations in the substantially transparent substrate). More particularly, as shown in  FIGS. 2   a-b , the aaPSM  10  includes a substantially transparent layer (e.g., quartz) and an opaque layer (e.g., chrome). The opaque layer is etched to form opaque regions  15  and alternating substantially transparent regions  13 , as shown in  FIG. 2   b . The substantially transparent regions  13  are further etched such that the aaPSM  10  has recesses  14  in the substantially transparent layer. In other words, the aaPSM  10  has substantially transparent regions  13  (which are un-etched) that alternate with etched recesses  14  between each opaque region  15 , as shown in  FIGS. 2   a-b . The effect of this structure when placed in a Stepper is to create light intensity of alternating polarity and 180° out of phase, as shown in  FIG. 2   c . This alternating polarity forces energy transmitted from the Stepper to go to zero, in theory, at opaque regions  15  while maintaining the same transmission of light at the alternating transparent regions  13  and recesses  14 . As a result, refraction is reduced through this region. In this regard, in recesses  14 , the following equation is satisfied:
 
 d=λ/ 2( n− 1)
 
where d is film thickness, n is refractive index at exposure wavelength, λ is exposure wavelength. Thus, it is possible to etch smaller features in a semiconductor wafer and use shorter exposure wavelengths. Since the photoresist layer on the semiconductor wafer ( FIG. 2   d ) is insensitive to the phase of the exposed light, the positive and negative exposed regions appear the same, while the zero region in between is clearly delineated. Thus, a sharper contrast between light (i.e., transparent) and dark (i.e., opaque) regions in the resulting photoresist layer of a semiconductor is obtained, thereby making it possible, in theory, to etch a more accurate image onto the semiconductor wafer.
 
     In practice, however, the aaPSM of  FIG. 2   b  has certain limitations which often preclude the possibility of transferring an accurate image from the aaPSM to a semiconductor wafer. In this regard, as feature sizes continue to get smaller, the intensity of light transmitted through recess  14  will often be less than the intensity of light transmitted through the unetched portions  13 . More particularly, referring to  FIG. 2   e , the light intensity is shown for each transmissive region of the aaPSM of  FIGS. 2   a  and  2   b . Length A represents the critical dimensions of recess  14  and Length B represents the critical dimensions of the unetched portion  13  of the substantially transparent layer. As can be seen, the critical dimensions of these features are asymmetrical (Length B−Length A). As a result, the intensity of light transmitted through the unetched portion  13  of the substantially transparent layer is greater than the intensity of light transmitted through the recess  14 . This imbalance of light intensity transmitted through these features often make it difficult, if not impossible, to write an accurate image on the a semiconductor wafer using the aaPSM shown in  FIG. 2   b.    
     It is known in the art of photomask design to etch highly anisotropic features (i.e., features etched more in one direction than in other directions) in aaPSMs, as shown in  FIGS. 3   a  and  4   a . Anisotropic features are typically formed by using a plasma reactor. In particular, it is known to use a fluorocarbon or hydrofluorocarbon etching gas and apply a radio frequency (“RF”) bias to the pedestal supporting the photomask. The RF bias creates a direct current (“DC”) bias in the plasma adjacent to the mask. The DC bias accelerates the ions towards the mask and the resulting etch is highly anisotropic with nearly vertical sidewalls. In addition to plasma etching techniques, wet etching techniques have been used to undercut features in the phase shift mask, as shown in  FIG. 3   b.    
     However, anisotropic features produce a waveguide effect during wafer printing which induces an aerial image intensity imbalance through focus on the wafer, as shown in  FIGS. 3   a - 3   h . For example, as shown in  FIGS. 3   c ,  3   e  and  3   g , aerial image intensity imbalance caused by aaPSM quartz features having sidewalls that have been anisotropically etched can result in a relative difference of exposure intensity at the wafer plane if the stepper is not in perfect focus. For example, where the stepper is −0.4 μm out of focus, the aerial image intensity of the energy transmitted through the aaPSM of  FIG. 3   a  is approximately 2.8 a.u. for shallow etched features and 2.2 a.u. for deep etched features, and approximately 3.5 a.u. for shallow etched features and 3.0 a.u. for deep etched features when in perfect focus (i.e., 0.0 μm). Any imbalance in aerial image intensity will result in an inaccurate image being written on the semiconductor wafer. In this regard, since the threshold energy needed to activate photoresist on the wafer is constant, any dissimilarity in intensity for adjacent features will produce a different final critical dimension for adjacent features on the wafer. As a result, the focus latitude required to obtain good pattern transfer from the photomask to the wafer is reduced. This impact on printability due to the waveguide effect has been shown in the prior art to be effectively eliminated by isotropically etching quartz trench features which were formed by anisotropic etching methods. 
     A known method for reducing aerial image intensity imbalance is to create isotropic trenches in conventional aaPSMs by utilizing: a dry plasma etching step to form an anisotropic trench; and thereafter, a wet hydrofluoric acid (HF) dip, as described in U.S. Patent Application Publication No. 2001/0044056 A1 to isotropically etch the anisotropic trench. As shown in  FIGS. 3   a ,  3   c ,  3   e  and  3   g , the aerial image intensity imbalance in this type of aaPSM is significantly reduced when the stepper is out of focus. Although useful for reducing aerial image intensity imbalance, the known methods (e.g., a dry etch followed by a wet etch) has significant drawbacks which have deterred photomask manufacturers from implementing this otherwise useful aaPSM. In particular, HF is known to be a very toxic and corrosive chemical which is hazardous to handle in a production environment. Thus, any alternative method that can achieve the same results without resorting to the use of this hazardous material is preferred. Additionally, HF requires separate processing equipment, and thus, makes the overall manufacture of photomasks more expensive and time consuming. Additionally, since HF is hazardous to the environment, it is necessary to dispose of it in a proper and lawful manner, which can also be costly and burdensome. Furthermore, the wet etch process is purely isotropic in nature and cannot be tuned to prevent excessive undercut and chrome liftoff. Excessive undercut and chrome liftoff is disadvantageous because it can cause defects. Thus, any process which can limit the need for undercutting chrome is preferred, especially where smaller feature sizes are used. An additional concern with respect to wet chemistry is the loading effects of dense to isolated patterned areas. In this regard, an isolated area&#39;s etch rates are effected by chemical dilution due to the extreme exposed areas, thereby making it difficult to control the etch time. Thus, wet etching techniques often result in excessive undercut in such exposed areas. Therefore, what is needed is an improved method for manufacturing aaPSMs without an aerial intensity imbalance which avoids using hazardous materials and is tunable to avoid excessive undercut and chrome liftoff and can minimize loading effects. 
     Although these prior art methods are useful in providing for balanced light intensity for some aaPSM designs, the additional step of undercutting the opaque regions of the photomask is both time-consuming and expensive. Accordingly, the overall number of aaPSMs which could be manufactured in a given time period is limited by these factors. Additionally, as feature sizes continue to get smaller, it will become increasingly difficult to undercut the chrome regions and the problems of chrome liftoff and excessive undercut will become increasingly prevalent. As a result, the use of aaPSMs will become less desirable and potentially obsolete. Moreover, the wet etching techniques of the prior art are known to be hazardous. 
     Thus, there is a long felt need for a new aaPSM and method for making the same which eliminates the need to undercut the opaque layer while at the same time provides for the transmission of balanced light intensities through the aaPSM. 
     Accordingly, it is an object of the present invention to provide an improved aaPSM which has a balanced aerial intensity which does not utilize hazardous materials. 
     It is another object of the present invention to provide an aaPSM for use in photolithography and for semiconductor fabrication to enhance resolution and depth of focus. 
     It is another object of the present invention to provide an improved aaPSM which has a balanced aerial intensity without excessive undercut and chrome liftoff. 
     It is another object of the present invention to solve the shortcomings of the prior art. 
     Other objects will become apparent from the foregoing description. 
     SUMMARY OF THE INVENTION 
     It has now been found that the above and related objects of the present invention are obtained in the form of a aaPSM having an intermediate film which balances the aerial intensity between two alternating recesses of different depths. 
     More particularly, the present invention relates to a blank photomask comprising: a photosensitive resist material layer; an opaque layer underlying the photosensitive resist material layer; a partially absorbing film layer underlying the opaque layer; and a substantially transparent substrate underlying the partially absorbing film layer. The partially absorbing film layer comprises a thickness which substantially balances an aerial image intensity of light transmitted through patterned regions when the blank photomask has been processed into an aaPSM. When processed, the patterned regions comprises at least one opening exposing the partially absorbing film layer and at least one light transmitting opening in which the partially absorbing film layer has been removed. 
     The present invention is also directed to a method for creating an aaPSM from the blank photomask described above and comprises the step of providing a blank photomask comprising a photosensitive resist material layer, an opaque layer underlying the photosensitive resist material layer, a partially absorbing film layer underlying the opaque layer, and a substantially transparent substrate underlying the partially absorbing film layer. The method further comprises the steps of forming in the blank photomask at least one opening which partially absorbs light, wherein the light absorbing opening exposes the partially absorbing film layer and forming at least one light transmitting opening in the blank photomask. When an aaPSM is formed by this method, the light transmitting opening exposes a portion of the substantially transparent substrate, wherein the partially absorbing film layer comprises a thickness which substantially balances an aerial image intensity of light transmitted through the at least one partially absorbing opening with light transmitted through the at least one light transmitting opening. 
     Additionally, the present invention is directed to an aaPSM comprising: a substantially transparent substrate having at least one light transmitting opening formed therein; a partially absorbing film layer covering portions of the substantially transparent substrate not defined by the at least one light transmitting opening; and a patterned layer of opaque material affixed to the partially absorbing film. In this aaPSM, the patterned layer exposes at least one portion of the underlying partially absorbing film layer, wherein the partially absorbing film layer comprises a thickness which substantially balances an aerial image intensity of light transmitted through the at least one exposed portion of the at least one partially absorbing film layer with light transmitted through the at least one light transmitting opening. 
     Additionally, the present invention is directed to another embodiment in which an aaPSM comprises a substantially transparent substrate; a partially absorbing film layer having at least one opening formed therein, wherein portions of the substantially transparent substrate underlying the at least one opening in the partially absorbing film layer are exposed; and a patterned layer of opaque material affixed to the partially absorbing film layer. In this embodiment, the patterned layer of opaque material has at least one opening which exposes underlying portions of the partially absorbing film layer, wherein the partially absorbing film layer comprises a thickness which substantially balances an aerial image intensity of light transmitted through the at least one opening in the opaque layer with light transmitted through at least one opening in the partially absorbing film layer. 
     Additionally, the present invention is directed to a method for manufacturing a semiconductor comprising the steps of: interposing an aaPSM between a semiconductor wafer and an energy source, wherein the an aaPSM comprises a substantially transparent substrate having at least one light transmitting opening formed therein; a partially absorbing film layer covering portions of the substantially transparent substrate not defined by the at least one light transmitting opening; and a patterned layer of opaque material affixed to the partially absorbing film. The patterned layer of the aaPSM exposes at least one portion of the underlying partially absorbing film layer, wherein the partially absorbing film layer comprises a thickness which substantially balances an aerial image intensity of light transmitted through the at least one partially absorbing opening with light transmitted through the at least one light transmitting opening. The method further comprises the steps of generating energy in the energy source; transmitting the generated energy through light transmitting opening in the substantially transparent substrate and the at least one exposed partially absorbing film layer; and etching an image on the semiconductor wafer corresponding to the light transmitting opening in the substantially transparent substrate and the at least one exposed partially absorbing film layer. 
     Additionally, the present invention is directed to a method for manufacturing a semiconductor comprising the steps of: interposing an aaPSM between a semiconductor wafer and an energy source, a substantially transparent substrate; a partially absorbing film layer having at least one opening formed therein, wherein portions of the substantially transparent substrate underlying the opening in the partially absorbing film layer are exposed; and a patterned layer of opaque material affixed to the partially absorbing film layer, the patterned layer of opaque material having at least one opening which exposes underlying portions of the partially absorbing film layer, wherein the partially absorbing film layer comprises a thickness which substantially balances an aerial image intensity of light transmitted through the at least one opening in the opaque layer with light transmitted through at least one opening in the partially absorbing film layer. This method further comprises the steps of generating energy in the energy source; transmitting the generated energy through the at least one opening in the partially absorbing film layer and the at least one exposed portion of the partially absorbing film layer; and etching an image on the semiconductor wafer corresponding to at least one opening in the partially absorbing film layer and the at least one exposed portion of the partially absorbing film layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and related objects, features and advantages of the present invention will be more fully understood by reference to the following, detailed description of the preferred, albeit illustrative, embodiment of the present invention when taken in conjunction with the accompanying figures, wherein: 
         FIG. 1   a  shows the equipment which can be used to make a semiconductor device from the aaPSM of the present invention; 
         FIG. 1   b  is flow diagram showing an example of the process for making a semiconductor device; 
         FIG. 2   a  shows a top view of a conventional aaPSM; 
         FIG. 2   b  shows a corresponding side view of the conventional aaPSM shown in  FIG. 2   a;    
         FIG. 2   c  shows the corresponding transmission of light through etched and un-etched regions of the substantially transparent layer of the conventional aaPSM of  FIGS. 2   a  and  2   b;    
         FIG. 2   d  shows the corresponding regions in a semiconductor wafer onto which the light is transmitted from the aaPSM shown in  FIGS. 2   a  and  2   b;    
         FIG. 2   e  demonstrates how the aaPSM of  FIGS. 2   a  and  2   b  experience an imbalanced light intensity when exposed to an energy source; 
         FIG. 3   a  shows a side view of an anistropically etched aaPSM; 
         FIG. 3   b  shows a side view of an aaPSM having anisotropic trenches that have been isotropically undercut using wet etching techniques; 
         FIG. 3   c  shows the aerial image intensity of the aaPSM of  FIG. 3   a  when the Stepper is in perfect focus; 
         FIG. 3   d  shows the aerial image intensity of the aaPSM of  FIG. 3   b  when the Stepper is in perfect focus; 
         FIG. 3   e  shows the aerial image intensity of the aaPSM of  FIG. 3   a  when the Stepper is out of focus; 
         FIG. 3   f  shows the aerial image intensity of the aaPSM of  FIG. 3   b  when the Stepper is out of focus; 
         FIG. 3   g  is a graph showing the aerial image intensity for deep and shallow etched trenches of the aaPSM of  FIG. 3   a  at different focus levels; 
         FIG. 3   h  is a graph showing the aerial image intensity for deep and shallow etched trenches of the aaPSM of  FIG. 3   b  at different focus levels; 
         FIG. 4   a  shows a cross-sectional view of conventional aaPSM having an anisotropic trench; 
         FIG. 4   b  shows a cross-sectional view of conventional aaPSM having an isotropic trench; 
         FIG. 5  shows a cross-sectional view of a photomask blank made in accordance with the present invention; 
         FIG. 6  shows a finished aaPSM made in accordance with the present invention using the photomask blank of  FIG. 5 ; 
         FIG. 7  shows one embodiment of the method for manufacturing the aaPSM shown in  FIG. 6 ; and 
         FIG. 8  shows another embodiment by which the photomask blank of  FIG. 5  is processed into a finished photomask. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed to an improved aaPSM and method for making the same. More particularly, the present invention utilizes a partially absorbent, intermediate film in a blank photomask to make an aaPSM which transmits balanced light intensities through each opening in the finished aaPSM. By utilizing a partially absorbent intermediate film in this manner, the step of undercutting the opaque layer of the aaPSM is no longer needed. 
     More particularly, referring to  FIG. 5  a blank photomask  31  made in accordance with the present invention is shown. The blank photomask  31  preferably includes four layers, but may include additional layers as needed or desired by the photomask maker. In particular, the blank photomask  31  includes a substantially transparent layer  33 , such as quartz. The next layer is a film  35  which is made from materials that are capable of both partially absorbing light and partially transmitting light. Since the photomask  31  will ultimately be processed into an aaPSM, the film should be tuned to have an index and thickness such that, when exposed to an energy source, the light transmitted therethrough will be partially absorbed so as to be in balance with light transmitted through the trenches  38  and  40  (described below) formed in the aaPSM. In this regard, the thickness of the film  35  should be selected in conjunction with the depth of the alternating recess and trenches of the aaPSM of the present invention so as to ensure that the light transmitted through said alternating recesses and trenches is approximately 180° out of phase. Further, the present invention may be used in conjunction with a lateral size bias or lateral size adjustment of either the recesses, trenches or opaque region. A variety of materials can be used as the film  35  of the present invention, including, but not limited to, MgF 2 , amorphous-Si, Si 3 N 4 , SiON, CrO, CrON, TiN, TiO, TaO, TaN, MoO, MoN, MoON, and any oxide, nitride, fluoride or oxynitride of any metallic element. The next layer is a substantially opaque layer  37  (e.g, chrome) capable of absorbing all (or most) light to which it is exposed. The opaque layer  37  may additionally include an anti-reflective layer, such as chrome oxide, if desired or needed. The next layer is a photosensitive material  39 . 
     To form the aaPSM of the present invention, a pattern defined by an electronic file is transferred to the blank photomask using conventional lithography tools, including, for example, E-beam and/or laser beam writing tools. In one embodiment, the laser source which is used operates at 365 nm, however, the present invention is not limited to this particular wavelength laser source and will work with a variety of different image sources as discussed herein. As described in detail below, the blank photomask  31  is etched to form an aaPSM  41  having types of transmissive regions: (1) an unetched, film recess  40  covering a corresponding unetched portion of the substantially transparent layer  33 ; and (2) a subtractively etched trench  38  etched in the substantially transparent layer  33 . These transmissive regions alternate between opaque regions on the photomask, as shown in FIG.  6 . 
     In one embodiment of the present invention to make the aaPSM of FIG.  6 . First, an opaque region is defined in the photomask. In one embodiment, this is accomplished by several steps. Referring to  FIG. 7 , the blank photomask of  FIG. 5  is provided, Step  1 . Predefined areas  32  in the photosensitive resist layer  39  of the blank photomask  31  are exposed to an energy source (e.g., a light source), Step  2 . Thereafter, the exposed photoresist  32  is developed (i.e., removed), thereby forming a recess  34  in the photosensitive resist layer  31 , Step  3 . Next, the portions of the opaque layer  37  underlying the recesses  34  are removed by conventional etching techniques (e.g., dry etching or chemical etching), Step  4 . Thereafter, the remaining photosensitive resist  39  overlying the undeveloped portions of the opaque layer  37  is removed, thereby exposing portions of the film  35  underlying the removed portions of the opaque layer  37 , Step  5 . As a result, the un-etched portions of the opaque layer  33  serve to block the exposure light during wafer printing, whereas the portions of the opaque layer which have been etched away define the region of the film  35  and substantially transparent layer  33  which allows exposure light from the Stepper to pass through. In essence, the first processing stage of the method of the present invention is used to form a modified version of a conventional binary photomask by patterning an opaque layer. 
     Next, alternating phase-shift features are formed in the substantially transparent layer  33 . This is also accomplished in several steps in this embodiment of the present invention. In particular, after Step  5  has been completed, the remaining portions of the opaque layer  33  and the uncovered portions of the film  35  are re-coated with photosensitive resist  39 , Step  6 . Predefined areas  36  in the photosensitive resist layer  39  of the photomask are exposed to an energy source, Step  7 . In this regard, these predefined areas preferably alternate with the recesses  40  wherein a portion of chrome separates each alternating recess  40  and predefined area. However, the present invention is not limited to PSMs which have alternating etched regions. It may also be used in any PSM which has etched regions of different depths, whether alternating or not. Next, the exposed areas  36  of the photosensitive resist layer  39  are developed (i.e., removed), Step  8 . Thereafter, the portions of the film  35  and the corresponding, underlying portions of the substantially transparent layer  33  that are no longer covered by photosensitive resist material  39  (or opaque material  33 ) are etched to a specified depth, Step  9 . As a result, a phase shifted, transmissive vertical trench  43  is formed in the substantially transparent layer  35 . Thereafter, the remaining photoresist  39  is removed, Step  10 . The result is an aaPSM having a vertical trench  38  alternating with unetched recess  40  of the substantially transparent layer that are covered by the film  35 . The specified depth and the thickness of film  35  are determined so that the light that passes through the unetched recess  40  is approximately 180° out of phase from light that passes through the vertical trench  38 . 
     It is noted, however, that the method for processing the aaPSM of the present invention is not limited to the particular processing steps. In this regard, the processing method could be modified so long as the same results are achieved. Additionally, it is noted that the aaPSM of the present invention can be modified to be etched to different depths. 
     For example, in another embodiment, the blank photomask of  FIG. 5  could be processed to form a modified aaPSM in which the substantially transparent layer does not need to be etched. Referring to  FIG. 8 , the blank photomask  31  could be processed to form a pattern in the mask having alternating trench depths wherein a first trench  51  is formed by simply removing the pre-defined portion of the opaque layer  37 , thereby exposing the underlying partially absorbing film  51 . Additionally a second trench  53  is formed by removing a second predefined portion of the opaque layer  37  and additionally removing the underlying portion of the partially absorbing film  35  which corresponds to the second predefined portion of the opaque layer  37 . Of course, additionally trenches may need to be formed depending on the overall design of the photomask. The partially absorbing film  35  should be of a sufficient thickness such that the phase delay of light transmitted through the first trench  51  and the second trench  53  approximately 180° out of phase and otherwise substantially equal. Depending upon the application, it may be desirable to use a thicker film and a shallower trench, or vice versa. The film thickness could be calculated through trial and error, through photomask modeling techniques or other known techniques. An additional advantage of this embodiment is that the trenches and recesses will have a natural etch stop for these manufacturing processes. 
     By making and using an aaPSM having a film as described herein and shown in  FIGS. 5-8 , there is zero asymmetry between the critical dimensions of the trench  38  and the unetched recess  40  of the substantially transparent layer  33  covered by the film  35 . As a result, the light intensity transmitted through these regions is balanced. Thus, it is possible to write an accurate image on a semiconductor wafer in a more efficient manner than was possible by prior art methods. In this regard, since the method of the present invention requires fewer etching steps (i.e., it does not require the step of undercutting the opaque layer), the overall output of aaPSMs over a given time period increases. 
     Now that the preferred embodiments of the present invention have been shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. For example, the present invention is not limited to the precise processing steps described herein. In this regard, the aaPSM of the present invention may be made with fewer or more processing steps, depending upon the equipment used and needs of the photomask maker. Further, the method of the present invention may also, for example, form all the unetched regions  40  in a series of processing steps, and form the etched regions  38  in a second series of processing steps. Thus, the present embodiments are therefor to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.