Abstract:
A manufacturing method of a semiconductor wafer includes forming a plurality of alignment trenches in the wafer substrate. A dielectric layer is formed over the substrate filling the trenches. A planarization process is performed to remove the dielectric layer above the substrate. A photolithograph process is subsequently performed to selectively remove the dielectric layer formed in the trenches in the alignment area.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to a method for fabricating a semiconductor wafer. More particularly, the present invention relates to a method for fabricating alignments marks on a semiconductor wafer, wherein alignment performance is enhanced. 
   2. Description of Related Art 
   The fabrication of semiconductor devices on a semiconductor wafer involves a transfer of a succession of patterns from photo masks onto the wafer. In order for the devices to perform properly, each circuit pattern must be formed aligning as precisely as possible with the circuit pattern that has been previously formed on the wafer. In other words, the overlay error must be minimized. 
   One alignment technique is known as the through-the-lens (TTL) alignment. The TTL alignment is characterized in that the alignment of a reticle or a mask and the wafer is effected through a projection optical system. The TTL alignment is achieved by illuminating the alignment marks on a reticle using a light source (for example, a 633 nm laser light), wherein each reticle alignment mark is composed of a series of parallel lines with a pitch. The image of the reticle alignment mark is projected by the projection optical system onto the wafer bearing alignment marks having similar alignment patterns of the reticle alignment mark. Portions of the light which are incident on the alignment pattern of the wafer alignment mark are reflected and detected. The portions of the light which are incident on the lines of the wafer alignment mark are strongly reflected, while the portions of the light that are incident between the lines are weakly reflected. Optimum alignment occurs when the alignment pattern on the reticle is properly aligned with the alignment pattern on the wafer. Typically, a minimum of two alignment marks is defined on each wafer, although increasing the number of alignment marks increases the alignment accuracy. However, the presence of these marks on the wafer reduces the available area for semiconductor elements. The increase of alignment marks for increasing the alignment accuracy is thereby hindered. 
   Alternatively, alignment marks, known as the ATHENA (Advanced Technology using High order Enhancement of Alignment) marks, are formed in the scribe line area provided between chips. In the ATHENA alignment technique, light is radiated onto the wafer. The radiated light is diffracted by the alignment marks, and the diffraction pattern is detected. The relative position of the wafer and the photomask is then adjusted accordingly. The quality of the diffracted light from the alignment mark is a directly related to the structure of the alignment mark, such as the material or the dimension of the mark. Since the alignment marks are formed in the scribe line area, there will be no unwanted loss of dies. Further, additional alignment marks may form per wafer, for example, two marks per shot or 8 to 18 marks per wafer, to improve the alignment accuracy. 
   Typically, each alignment mark comprises a topographical pattern, such a plurality of grooves and lands, which can be formed by, for example etching a controlled depth into wafer. The etching of the wafer creates a step height in the wafer. Once the alignment mark is formed on a wafer, it will be used for position detection in subsequent processes. However as the wafer has undergone various processes of forming circuit patterns thereon, the integrity of the alignment mark on the wafer is compromised. For example, some of the intermediate processing steps of forming circuit patterns on the wafer, such as chemical mechanical polishing (CMP) or deposition of thick or opaque layer, can damage or distort the alignment marks on the wafer or bury the marks under opaque layers. 
   When a chemical mechanical polishing (CMP) process, which is planarization technique, is performed to planarize a dielectric layer, not only the dielectric layer over the semiconductor element area is planarized, the dielectric layer over the alignment mark in the scribe line area is also planarized. Accordingly, the step height of the alignment mark is destroyed. If polysilicon or metal silicide or metal layers, which are opaque, are further formed thereon, the alignment mark will become invisible. 
   When the alignment mark has low reflectivity and small step height, the mark is difficult to be detected. Further, since additional layers may have deposited on the mark, the reading of the mark is interfered, for example the contrast of the image of the mark is poor and the reading of the image is noisy. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, the present invention provides a method for fabricating a semiconductor wafer, wherein alignment accuracy and overlay error are improved. 
   The present invention also provides a method for fabricating a semiconductor wafer, wherein the removal of the alignment mark subsequent to a planarization process can be prevented. 
   The present invention provides a method for fabricating an alignment mark, wherein the step height of the alignment mark is preserved and replicated in subsequently formed layers. 
   According to one aspect of fabrication method of a semiconductor wafer of the present invention, a substrate having at least an alignment area is provided. The alignment area further includes at least one alignment mark in a form of trenches formed in the substrate. A dielectric layer is deposited covering the substrate and filling the trenches. Thereafter, a portion of the dielectric layer above the substrate is removed. A photolithograph and etching process is performed to selectively remove the portion of the dielectric layer in the trenches. The removal of the portion of the dielectric layer includes performing a planarization process, for example. 
   According to the fabrication method of a semiconductor wafer of the present invention, a photomask is provided for the photolithograph and etching process, wherein the pattern on the photomask corresponds to the dimension and the position of the alignment mark. 
   According to the fabrication method of a semiconductor wafer of the present invention, photo exposure of the photolithograph and etching process is only performed at the alignment area. 
   According to another aspect of fabrication method of a semiconductor wafer of the present invention, a substrate having at least an alignment area and a device area is provided. A plurality of first trenches and a plurality of second trenches are formed in the substrate of the alignment area and the device area, respectively. A dielectric layer is formed over the substrate and filling the first and the second trenches. Thereafter, a planarization process is performed to remove the dielectric layer above the substrate. A photolithograph and etching process is subsequently performed to selectively remove the dielectric layer in the first trenches. 
   According to the fabrication method of a semiconductor wafer of the present invention, a photomask is provided for the photolithograph and etching process, wherein the pattern on the photomask corresponds to the dimension and position of the first trenches. 
   According to the fabrication method of a semiconductor wafer of the present invention, photo exposure of the photolithograph and etching process is performed selectively at the first trenches. 
   In accordance to the fabrication method of a semiconductor wafer of the present invention, since the dielectric layer formed in the alignment mark is removed, the step height of the alignment mark is preserved. 
   Further, since the pattern of the photomask, used in selectively removing the dielectric layer in the alignment mark, corresponds to the dimension and position of the alignment mark, the photomask is simple and thus inexpensive. 
   Moreover, only limited photo exposures are performed at selective locations, for example, only at the locations of the alignment mark, the throughput can be improved. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIGS. 1A to 1F  are schematic, cross-sectional views showing selected process steps for fabricating a semiconductor wafer including an alignment area and a device area according to one embodiment of the present invention. 
       FIG. 2  is a schematic cross-sectional view of the alignment area with the deposition of material layers over the alignment area subsequent to the process step depicted in  FIG. 1D . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A to 1F  are schematic, cross-sectional views showing selected process steps for fabricating a semiconductor wafer including an alignment area and a device area according to one embodiment of the present invention. 
   Referring to  FIG. 1A , a substrate  100  of a semiconductor wafer is provided, wherein the substrate  100  comprises at least a device area  100   a  and an alignment area  100   b . The substrate  100  is a silicon substrate, for example. In one aspect of the invention, the alignment area  100   b  is configured in regions include but not limited to the scribe line area of the wafer. 
   An oxide layer  102  is then formed on the substrate  100 . The oxide layer  102  is either grown by an oxidation method or deposited by a chemical vapor deposition method on the substrate  100  to a thickness of about 150 Å to 200 Å, for example. The oxide layer  102  is, for example, a pad oxide layer serves to protect the substrate  100  of the subsequent processes. A dielectric layer  104  is further formed over the oxide layer  102 . The dielectric layer  104  that preferably contains nitride, may serve as an etch stop layer. The thickness of the dielectric layer  104  is about 1100 Å to 1500 Å, for example. 
   Referring to  FIG. 1B , a patterned photoresist layer  106  is formed over the substrate  100 , exposing a part of device area  100   a  where patterned feature definitions are going to be formed and a part of the alignment area  100   b  where alignment marks are going to be formed. Using the patterned photoresist layer  106  as an etching mask, the dielectric layer  104 , the oxide layer  102  and the substrate  100  in the alignment area  100   b  are etched to form a plurality of alignment trenches  108   a  in the substrate  100 . These alignment trenches  108   a  basically constitute an alignment mark  108  used in the subsequent processes for aligning photomasks with the wafer. In one aspect of the invention, the dielectric layer  104 , the oxide layer  102  and the substrate  100  in the device area  100   a  are concurrently removed to form at least a patterned feature definition in the substrate  100 . For example, an isolation trench  112  is formed in the substrate  100  for defining an active area in the device area  100   a . The etching is, for example, an anisotropic etching such as a reactive ion etching. In one embodiment, the depth of the trenches  108   a ,  112  is about 2900 Å to 3200 Å. 
   As shown in  FIG. 1C , the patterned photoresist layer  106  is removed. Thereafter, one or more layers may deposit to line the trenches  108   a ,  112 . In one aspect of the invention, the one or more liner layers  116  may include, for example an oxide layer, a nitride layer and/or a silicon oxynitride layer. A total thickness of the one or more liner layers  116  is about 150 Å to about 275 Å, for example. In the case that the one or more liner layers  116  include an oxide layer and a nitride layer, the oxide layer is formed by growing in a furnace at about 1000° C. to 1200° C. to about 170 Å to 220 Å thick, for example, while the nitride layer of about 50 Å to 70 Å thick is formed by treating the oxide layer in ambient N 2  at an elevated temperature or performing a nitrogen atom implant process, followed by an annealing process. 
   Still referring to  FIG. 1C , a dielectric layer  118 , typically a thick oxide layer, is blank deposited over the substrate  100  to a thickness of about 6000 Å to 7000 Å, filling the isolation trench  112  in the device area  100   a  and the alignment trenches  108   a  in the alignment area  100   b . For example, the dielectric layer  118  may form by performing a high density plasma chemical vapor deposition (HDPCVD), followed by an annealing process at about 1000° C. to 1100° C. for densification purposes. As seen in  FIG. 1C , although the alignment trenches  108   a  is covered by the dielectric layer  118 , the alignment mark  108  (for example, the step height) is preserved in the dielectric layer  118 . 
   Continuing to  FIG. 1D , a chemical mechanical planarization (CMP) process is performed. The CMP process is a global planarization process that planarizes the dielectric layer  118  in both the device area  100   a  and the alignment area  100   b  until the dielectric layer  104  is exposed. As can be seen, CMP removes the step height in the alignment mark  108 . 
   In the case that the alignment mark  108  as shown in  FIG. 1E  is continued to use for fabricating subsequent layers of the integrated circuits, layers, such as a diffusion layer or a gate layer, etc., will be deposited thereon. Referring to  FIG. 2 , wherein  FIG. 2  is a schematic cross-sectional view of the alignment area after the deposition of material layers over the alignment area subsequent to the process step depicted in  FIG. 1D . After the CMP process, a polysilicon layer  202 , a silicon nitride layer  204 , for example, may form over the alignment mark  108 . Although the step height has been removed, the alignment mark  108  may still be vaguely visible due to an image contrast between the silicon substrate  100  and the oxide dielectric layer  118  deposited in the alignment mark  108 . However, with the subsequent depositions of an oxide layer  206 , a polysilicon layer  208  and a metal silicide layer  210 , for example, the alignment mark  108  becomes completely invisible because some of these layers are opaque and the step height has been removed during the planarization process. 
   Accordingly, the present invention provides a selective photolithograph process subsequent to the planarization process. The photomask used in the selective photolithograph process has a pattern that corresponds to the dimension and position of the alignment mark. As depicted in  FIG. 1E , using the photomask  110 , a patterned photoresist  120  is formed over the substrate  100  by performing photo exposures selectively to the alignment mark  108  area, for example just 8 to 16 exposure locations Basically, the patterned photoresist  120  is designed to expose only the alignment mark  108  area. 
   Using the patterned photoresist  120  as a mask, the dielectric layer  118  formed inside the alignment mark  108  is removed. The dielectric layer  118  is removed by methods including wet etching, with a buffered oxide etch solution comprising ammonium fluoride and hydrofluoric acid, for example. The removal of the dielectric layer  118  in the alignment trenches  108   a  again creates a step height in the alignment mark  108 . 
   Referring to  FIG. 1F , the photoresist layer  120  is stripped eventually. Thereafter, the dielectric layer  104  disposed over the substrate  100  is removed, for example, using hot phosphoric acid and the underlying oxide layer  102  is removed, for example, with a hydrofluoric acid solution. 
   Thereafter, by utilizing the alignment mark  108  formed in the abovementioned process steps, subsequent layers used to form the integrated circuits are formed over the wafer substrate  100 . For example, in one embodiment, a conductive layer  122  and a dielectric layer  124  may sequentially form over the substrate  100  of both the device area  100   a  and the alignment area  100   b . The conductive layer  122  may include a polysilicon layer, for example. The dielectric layer  124  includes but not limited to a nitride layer. Although the alignment mark  108  is covered by these material layers, the step height and therefore, the alignment mark  108 , is replicated in these layers, for example, the conductive layer  122  and the dielectric layer  124 . The replicated alignment mark  108  can be used for aligning subsequent layers, including metallic layers that are opaque. Since the step height of the alignment mark  108  is preserved, the alignment of the subsequent layers can be accomplished and the alignment performance is enhanced. 
   In accordance to the present invention, since the step height of the alignment mark  108  is preserved, the reading of the alignment mark is facilitated and the alignment of the subsequent layers, including opaque layers, can be achieved more precisely. Therefore, the alignment accuracy and stability can be improved, and the overlay error can be better controlled. 
   Further, the photomask used in the selective photolithograph process is for exposing the alignment mark  108  area. The pattern of the photomask may simply correspond to the dimension and the position of the alignment mark. Therefore, the design of the photomask is simple and inexpensive. Further, only limited photo exposures are performed at selective locations, for example, only at the locations of the alignment mark, the throughput can be improved. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the present invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.