Abstract:
A method of fabricating a suspended structure. First, a substrate including a photoresist layer hardened by heat is provided. Subsequently, the hardened photoresist layer is etched so as to turn the photoresist layer into a predetermined edge profile. Thereafter, a structure layer is formed on parts of the substrate and parts of the photoresist layer. Next, a dry etching process is performed so as to remove the photoresist layer, and to turn the structure layer into a suspended structure.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of fabricating a suspended structure, and more particularly, to a method of using a hardened patterned photoresist layer to be a sacrificial layer, and adjusting the sidewall shape of the sacrificial layer for fabricating a suspended structure. 
   2. Description of the Prior Art 
   Micro-electromechanical systems (MEMS) are modern technology, which coordinate electrical circuits and mechanics. MEMS have been applied to fabricate devices with both electrical circuits and mechanical elements, such as pressure sensors, accelerators, and micro-microphones. The suspended structure is applied in many MEMS devices such as a MEMS switch, an inkjet nozzle, and micro-microphones, etc., but the method of fabricating a suspended structure according to the prior art still has many limitations. 
   Please refer to  FIG. 1  and  FIG. 2 .  FIG. 1  and  FIG. 2  show the method of fabricating a suspended structure  18  by using a wet etching process according to the prior art. As shown in  FIG. 1 , a substrate  10  is first provided, and the upper surface of the substrate  10  includes a patterned structure layer  14  thereon. As shown in  FIG. 2 , a patterned photoresist layer (not shown in the figure) is formed on the substrate  10  and the structure layer  14 , and a wet etching process is substantially performed. During the wet etching process, a part of the substrate  10  is etched through the opening of the patterned photoresist layer by an etching solution, and a suspended structure  18  is therefore formed. After that, the patterned photoresist layer is removed. 
   In general, the material of the substrate  10  is silicon, and the wet etching process uses a potassium hydroxide (KOH) solution as an etching solution to remove the substrate  10  so as to form the suspended structure. Because the prior art method utilizes the wet etching process to form a hole under the suspended structure  18 , the material of the structure layer  14  must be limited to particular metals that can sustain the etching solution. Otherwise, other material layers, such as a patterned photoresist layer, must be formed on the structure layer  14  to protect the structure layer  14 . Furthermore, the prior art method cannot produce various suspended structures  18  having different inclined angles, so it cannot satisfy some required designs of the products. 
   On the other hand, the etching solution of the wet etching process often damages the structure layer  14 , or results in ion pollution in the components. For example, the strength of the suspended structure  18  may be decreased by the surface tension of the KOH solution, so the suspended structure  18  collapses easily and sticks to the substrate  10 . The substrate  10  and the structure layer  14  may be polluted by potassium ions (K + ) in the KOH solution, which decreases the whole yield of the fabricating process. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a method of fabricating a suspended structure that utilizes a hardened patterned photoresist layer to be a sacrificial layer so as to solve the problems of the prior art. 
   According to the claimed invention, the present invention provides a method of fabricating a suspended structure. Accordingly, a substrate is first provided. The substrate comprises a first patterned photoresist layer thereon. Subsequently, the first patterned photoresist layer is heated so as to harden the first patterned photoresist layer and to round an edge of the first patterned photoresist layer. Next, the first patterned photoresist layer is etched so that a sidewall shape of the first patterned photoresist layer is adjusted. A second patterned photoresist layer is thereafter formed on the substrate. The second patterned photoresist layer exposes a part of the first patterned photoresist layer and a part of the substrate. Furthermore, a structure layer is formed on the substrate, the first patterned photoresist layer, and the second patterned photoresist layer. Next, a lift off process is performed to remove the second patterned photoresist layer and the structure layer above the second patterned photoresist layer. Thereafter, a dry etching process is performed to remove the first patterned photoresist layer so that the structure layer turns into the suspended structure. 
   According to another claimed invention, a method of fabricating a suspended structure is provided. First, a substrate is provided. The substrate comprises a first patterned photoresist layer thereon. Subsequently, the first patterned photoresist layer is heated so as to harden the first patterned photoresist layer. Furthermore, an etching mask is formed on the first patterned photoresist layer and the substrate. The etching mask exposes a part of the first patterned photoresist layer. Next, the first patterned photoresist layer not covered by the etching mask is etched so that the first patterned photoresist layer has a predetermined shape. The etching mask is thereafter removed. Moreover, a structure layer is formed on the substrate and the first patterned photoresist layer. A dry etching process is next performed to remove the first patterned photoresist layer so that the structure layer turns into the suspended structure. 
   Because the present invention uses a hardened patterned photoresist layer to be a sacrificial layer, the sidewall shape of the suspended structure can be adjusted to satisfy a variety of products, and the patterned photoresist layer can be removed by a dry etching process. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  and  FIG. 2  show the method of fabricating a suspended structure by using wet etching process according to the prior art. 
       FIG. 3  through  FIG. 13  illustrate a method of fabricating a suspended structure in accordance with a preferred embodiment of the present invention. 
       FIG. 14  shows a top view of the suspended structure in  FIG. 10 . 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 3  through  FIG. 14 .  FIG. 3  through  FIG. 13  illustrate a method of fabricating a suspended structure  40  in accordance with a preferred embodiment of the present invention, and  FIG. 14  shows a top view of the suspended structure  40  in  FIG. 10 . As shown in  FIG. 3 , a substrate  30  is first provided, and a first patterned photoresist layer  32  is included on the front surface of the substrate  30 , where the substrate  30  can be a silicon wafer or a SOI substrate, but is not limited to this. Substantially, as shown in  FIG. 4 , the first patterned photoresist layer  32  is heated. The heating process  32  can use an oven or heating plate to bake the substrate  30  and the first patterned photoresist layer  32 . The solution in the first patterned photoresist layer  32  is therefore decreased. As a result, the attaching ability of the first patterned photoresist layer  32  is increased, the first patterned photoresist layer  32  is hardened, and an edge of the first patterned photoresist layer  32  is simultaneously rounded. 
   As shown in  FIG. 5 , an etching mask  34  is thereafter formed on the substrate  30  and the first patterned photoresist layer  32 . The etching mask  34  exposes a part of the first patterned photoresist layer  32 . For example, an edge of the first patterned photoresist layer  32  that should be adjusted is exposed. The etching mask  34  can include materials, such as metal, silicon, oxide or nitride, depending on the processes&#39; requirements. As shown in  FIG. 6 , an etching process is performed to etch a part of the first patterned photoresist layer  32  through the opening of the etching mask  34  so that the first patterned photoresist layer has a vertical sidewall. It should be noted that the etching process could be adjusted according to the required component structure of different products. For instance, the process of etching the first patterned photoresist layer  32  can include a plasma etching process or a reactive ion etching (RIE) process, and the shape of the first patterned photoresist layer  32  can be changed by adjusting these process parameters. In addition to the vertical sidewall shown in  FIG. 6 , the first patterned photoresist layer can include an outwardly inclined sidewall shown in  FIG. 7 , an inwardly inclined sidewall shown in  FIG. 8 , or even an arc sidewall according to the present invention shown in  FIG. 9 . 
   As shown in  FIG. 10  and  FIG. 14 , the etching mask  34  is removed after the required sidewall of the first patterned photoresist layer  32  is shaped, and a second patterned photoresist layer  36  is thereafter formed on the substrate  30 . The second patterned photoresist layer  36  exposes a part of the substrate  30  and a part of the first patterned photoresist layer  32 , where the exposed regions are at the predetermined positions of the suspended structure&#39;s structure layer. As shown in  FIG. 11 , after the second patterned photoresist layer  36  is formed, a structure layer  38  is formed on the substrate  30 , the first patterned photoresist layer  32 , and the second patterned photoresist layer  36 . The structure layer  38  in this embodiment is a metal layer formed by deposition process, but the material of the structure layer  38  is not limited to this. The structure layer  38  might include kinds of metals, single crystal silicon, amorphous silicon polysilicon, or other proper materials. Different process, such as a chemical vapor deposition (CVD) process, a physical vapor deposition (PVD) process or a coating process, can be performed according to the needed materials of the structure layer  38 . 
   Next, as shown in  FIG. 12 , a lift off process is performed. The lift off process can remove the second patterned photoresist layer  36  by wet etching, and parts of the structure layer  38  above the second patterned photoresist layer  36  can also be removed accordingly. Simultaneously, parts of the structure layer  38  above the substrate  30  and parts of the structure layer  38  above the first patterned photoresist layer  32  remain. Furthermore, as shown in  FIG. 13 , a dry etching process, such as a dry chemical etching process or a RIE process, is performed to remove the first patterned photoresist layer  32 . Therefore, the remaining structure layer  38  above the substrate  30  becomes the suspended structure  40 . 
   In summary, because the present invention uses a hardened patterned photoresist layer to be a sacrificial layer of the suspended structure, the patterned photoresist layer can be removed by a dry etching process. It prevents a surface tension of a wet etching solution from destroying the strength of the suspended structure, and protects the suspended structure from collapsing and sticking to the substrate. In addition, since the heated photoresist is round on its edge, the follow-up structure layer can closely cover on the heated photoresist, and the present invention prevents a bad covering condition in the corners. Therefore, the suspension structure will not be broken easily. Accordingly, the method of the present invention provides a better protection for the suspended structure, and increases the whole yield of the fabricating process. 
   On other hand, the present invention can easily control the shape of the photoresist by various etching processes, so the sidewalls of the suspended structure can include at least two different inclined angles. As a result, the sidewall shape of the suspended structure can be easily adjusted to satisfy a variety of products, and the cost of the manufacturing process is reduced. Furthermore, since the method of the present invention utilizes the patterned photoresist layer to be the sacrificial layer, and the patterned photoresist layer is removed by a dry etching process, the material of the structure layer is no longer to be limited to particular metals that can sustain the KOH solution. Any metal or material, which can deposit and coat on the surface of the photoresist layer, can be applied to the structure layer in the present invention. Therefore, the suspended structure can include more variations, and satisfy more designs of the products due to the various materials of the structure layer. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.