Patent Publication Number: US-6342426-B1

Title: Method for protecting stepper alignment marks

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semi-conductor manufacturing process. More particularly, it relates to a method for protecting stepper alignment marks. 
     2. Description of the Prior Art 
     Alignment marks are indispensable for the photolithographic processing of semiconductor wafers. Alignment marks help ensure that the masks which are necessary for exposing the photoresist on the semiconductor wafer can be aligned exactly. This presupposes that the alignment marks have an exact assignment to the structures to be fabricated on the semiconductor wafer and, furthermore, offer a sufficient topographical contrast to be able to be reliably identified. 
     For power IC products, damage to the scribe line due to metal etching process-related problems usually cannot be avoided. However, along the scribe, there are many stepper alignment marks, which are used for stepper alignment while exposing the wafer. Due to this damage, these marks cannot be used for alignment causing difficulties with the photo process. 
     In order to ensure thorough etching in the metal etching process, when a portion requiring lower etching rates is finished, an underlying film beneath an earlier completed etching portion will also be etched, resulting in over-etching. Since the thickness of the metal layer in power IC products is thicker (3 μm) than general products, under normal process circumstances, the scribe line will be damaged after the metal etching process. The marks cannot be used for alignment on subsequent photolithographic processes. 
     FIG. 1 is a diagrammatic top view of a substrate after etching a metal layer thereon. The substrate  10  is divided into the integrated circuit area  12  and the scribe line  14 , and the alignment mark  16  is situated on the scribe line  14 . As can be seen, the scribe line is damaged after the metal etching process, thus rendering the alignment marks indistinct. The consequence is an increased outlay on post processing on account of the mask&#39;s misalignment. Alignment may even become impossible. This problem is becoming more prevalent as the result of ever-decreasing structure widths and increased demands for alignment accuracy. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a method for protecting stepper alignment marks. Since process-related damage only occurs after metal etching, in order to protect all the alignment marks during the metal etching process, a protection pattern (mark) is added right above the mark needed to be protected on the metal photo layer mask. By doing this, the photoresist remains after the metal photo layer and the photoresist is used to protect all the alignment marks from the metal etching. After metal dry etching, the photoresist remaining on top of the alignment marks is removed and the undamaged marks underneath can be used for stepper alignment. 
     According to the present invention, a method for protecting stepper alignment marks suitable for a substrate with an alignment mark on a scribe line and a metal layer that will be etched includes the following steps. First, a photoresist layer is formed over the metal layer. Next, a photo mask which has a predefined photo mask pattern for transfer to the metal layer is provided, and a pattern protecting the alignment mark is added to the photo mask pattern. Then, a photolithographic process is performed with the photo mask pattern on the photoresist layer to form the desired transferring photoresist mask to the metal layer and the protective photoresist mask for protecting the alignment marks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description, given by way of examples and not intended to limit the invention to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic top view of a substrate after etching a metal layer thereon. 
     FIGS. 2A through 2E illustrate, in cross-section, the process of a preferred embodiment according to the present invention. 
     FIG. 3 is a diagrammatic top view after performing the processes for protecting stepper alignment marks, in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method for protecting stepper alignment marks. 
     This embodiment begins by providing a substrate  20 , as shown in FIG.  2 A. The substrate  20  includes an integrated circuit area  22  and a scribe line  24 . The substrate  20  also has an alignment mark  26  on the scribe line  24  and a metal layer  28  that will be etched. The substrate  20  is selected from the group consisting of glass, ceramic, metal, semiconductor, polymer and combinations thereof. Materials used for alignment marks depend on the individual process. If the marks are formed after an oxide process, the marks will be oxide. If the marks are formed after a polysilicon process, the marks will be polysilicon. All the marks are formed after the etching process. The metal layer  28  can be comprised of, for example, metal, an alloy, preferably comprised of copper, aluminum, or its alloy. 
     Next, as shown in FIG. 2B, a photoresist layer  30  is formed over the metal layer  28 . The photoresist layer  30  is preferably formed by a spin coating process. 
     With reference to FIG. 2C, a photo mask  32  with a predefined photo mask pattern  34  for transfer to the metal layer  28  is provided. A pattern  36  protecting the alignment marks is added to the photo mask pattern  34 . 
     Referring to FIG. 2D, a photolithographic process is performed with the photo mask pattern  34  on the photoresist layer  30  to form the desired transferring photoresist mask  30   a  to the metal layer and the protective photoresist mask  30   b  for protecting the alignment mark. Photolithographic process includes generally exposing and developing technologies, and exposing technologies include stepperexposing technology. 
     Finally, the photoresist mask  30   a  and  30   b  are used as a mask for etching the metal layer  28  to form the desired etching pattern, then the photoresist mask  30   a  and  30   b  are removed. As a result, as shown in FIG. 2E, wherein metal layer  28   a  is the desired transferring pattern, and metal layer  28   b  is the protective pattern for protecting the alignment mark. The etching process is preferably an anisotropical dry etching or an isotropical wet etching process. 
     FIG. 3 is a diagrammatic top view after performing the processes for protecting stepper alignment marks, in accordance with a preferred embodiment of the present invention. The substrate  20  is divided into the integrated circuit area  22  and the scribe line  24 , and the alignment mark  26  is situated on the scribe line  24 . As can be seen, due to the alignment mark  26  is protected with metal layer  28   b  as the protective pattern, the alignment mark  26  is still clear. 
     While the invention has been particularly shown and described with the reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.