Abstract:
The invention provides a method for forming a thin film resistor, which comprises the following steps: providing an insulator substrate; forming a patterned conductive layer over the insulator substrate by a non-photolithographic method; forming a thin film resistive layer on the patterned conductive layer and the insulator substrate; patterning the thin film resistive layer by photolithography. Using the method for forming a thin film resistor in accordance with the invention, the fabrication costs of the thin film resistor can be lowered.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for forming a thin film resistor. 
     2. Description of the Related Art 
     In the art of microelectronics fabrications, such as integrated circuit microelectronics fabrications and hybrid circuit microelectronics fabrications, thin film resistors are used as passive electrical circuit elements. 
     When thin film resistors are used in hybrid circuit microelectronics fabrications, a typical method for forming the resistors comprises the steps as follows: First, a layer of thin film resistive material and a layer of conductive material are formed, in this order, over an insulator substrate such as a glass insulator substrate or a ceramic insulator substrate. Next, the conductive material and then the resistive material are patterned by photolithography to form a patterned conductive layer and a patterned resistive layer. Next, the resistance of the patterned resistive layer is determined through laser trimming and the insulator substrate is then parted to form discrete thin film resistors. 
     The above-mentioned method of the prior art has the following disadvantages. Since the patterned conductive layer and the patterned resistive layer are formed by photolithography, and costs of photolithographic apparatus and materials are high, the fabrication costs of discrete thin film resistors are high. In the prior art method for forming the patterned conductive layer and the patterned resistive layer by photolithography, an insulator substrate of enhanced surface flatness and finish is typically required in a spin-coating process of a photoresist layer, in order to ensure the photoresist layer having an uniform thickness and provide an adequate registration of the insulator substrate within photolithographic apparatus. Generally, an aluminum oxide blank substrate with a purity of 99.6% is required, and it needs to be polished in order to enhance its surface flatness and finish. Compared with a conventional aluminum oxide substrate already provided with scribe lines, the aluminum oxide blank substrate cannot be cut simply along the scribe lines at a low cost but must be cut using laser, and thus further increases the fabrication costs of discrete thin film resistors. 
     SUMMARY OF THE INVENTION 
     Therefore, it is the object of the invention to provide a method for forming a thin film resistor solving all the above-mentioned problems. 
     The method for forming a thin film resistor in accordance with the invention comprises the following steps: providing an insulator substrate; forming a patterned conductive layer over the insulator substrate by a non-photolithographic method; forming a thin film resistive layer on the patterned conductive layer and the insulator substrate; applying photoresist by a known dry film process or a spray coating process in place of a spin coating process and then patterning the thin film resistive layer by photolithography. Since the dry film process and the spray coating process can be applied to an uneven surface of substrate, thin film resistors can be formed on a conventional aluminum oxide substrate already provided with scribe lines and can be separated by simply cutting along the scribe lines with low costs. Thus, the costs of raw materials and the manufacturing costs can be lowered. In this case, the insulator substrate can be a glass insulator substrate or a ceramic insulator substrate. A leveling layer can be formed on the insulator substrate before forming the patterned conductive layer over the insulator substrate. 
     The above-mentioned non-photolithographic method can be a screen printing method. The patterned conductive layer can be made from a material comprising silver, silver alloys, gold, gold alloys, copper, copper alloys, palladium, palladium alloys, nickel, or nickel alloys. In addition, the thin film resistive layer can be made from a material comprising tantalum nitride resistive materials, tantalum silicide resistive materials, tantalum-chromium alloy resistive materials, nickel-chromium alloy resistive materials, chromium silicide resistive materials, or higher order alloys of the aforementioned resistive materials. The thin film resistive layer can be formed by a method selected from the group consisting of thermally assisted evaporation methods, electron beam assisted evaporation methods, physical vapor deposition methods, chemical vapor deposition methods, or plasma enhanced chemical vapor deposition methods. 
     Laser trimming can adjust the resistance of the patterned thin film resistive layer. In addition, the resistance of the thin film resistive layer can be adjusted during the photolithographic patterning of the thin film resistive layer. 
     Using the method of the invention, since a non-photolithographic method is used in place of a conventional photolithographic method to form the thin film resistive layer, the fabrication costs of a thin film resistor is lowered. Furthermore, an aluminum oxide substrate with a purity of 99.6%, used in conventional method, is not required in the invention. Instead, an aluminum oxide substrate with a purity of 96% can be used, which greatly lowers the fabrication costs of the discrete thin film resistors. 
     The above and other objects, advantages, and features of the invention will become apparent from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic view showing a scribed insulator substrate used in the invention; 
     FIGS. 2 to  6  are cross-sectional views showing the successive steps of the method for forming a thin film resistor in accordance with the invention; and 
     FIG. 7 is a schematic view showing a portion corresponding to the cross-sectional view of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment in accordance with the invention will hereinafter be described with reference to the drawings. 
     Referring to FIG. 1, an insulator substrate  10  used in the invention has scribe lines including lengthwise scribe lines b 1 , b 2 , b 3 , and b 4 , and horizontal scribe lines a 1 , a 2 , and a 3 . The scribe lines can be formed in the insulator substrate  10  through any known method such as physical scribing methods employing scribes or other cutting tools, or scribing methods employing laser beams. The insulator substrate  10  of the invention can be, for example, a glass insulator substrate, or a ceramic insulator substrate. 
     Next, a leveling layer  12  can be formed on the insulator substrate  10  as shown in FIG. 2, in order to roughly planarize the upper surface of the insulator substrate  10  and fill in pits on the surface of the substrate. The leveling layer  12  can be a silicon oxide layer or a silicon nitride layer formed by any one of the known thin film formation methods such as thermally assisted evaporation methods, electron beam assisted evaporation methods, physical vapor deposition methods, chemical vapor deposition methods, or plasma enhanced chemical vapor deposition methods. Alternatively, the leveling layer  12  can be a glaze layer formed by any one of the known thick film methods. The material used to form the glaze layer can include silicon oxide, aluminum oxide, and titanium oxide. It is to be noted that the step of forming the leveling layer  12  is not absolutely necessary, but a method for forming a thin film resistor with this step can produce a resistor with improved quality. Furthermore, a glazed substrate can be used as the insulator substrate  10  of the invention and then it is processed as follows. 
     A patterned upper conductive layer  14  is formed on the leveling layer  12 , as shown in FIG. 3, through a non-photolithographic method such as a screen printing method, and a corresponding patterned lower conductive layer  16  is formed on the lower surface of the insulator substrate  10 . As shown in FIG. 3, the patterned upper conductive layer  14  includes discrete upper leads  14   a ,  14   b , and  14   c , and the patterned lower conductive layer  16  includes discrete lower leads  16   a ,  16   b , and  16   c . The patterned upper conductive layer  14  and the patterned lower conductive layer  16  of the invention can be made using materials that include silver, silver alloys, gold, gold alloys, copper, copper alloys, palladium, palladium alloys, nickel, or nickel alloys. 
     Subsequently, a thin film resistive layer  18  is formed on the patterned upper conductive layer  14  and the leveling layer  12  as shown in FIG.  4 . The thin film resistive layer  18  of the invention can be made using a material comprising tantalum nitride resistive materials, tantalum silicide resistive materials, tantalum-chromium alloy resistive materials, nickel-chromium alloy resistive materials, chromium silicide resistive materials, or higher order alloys of the aforementioned resistive materials. In addition, the thin film resistive layer  18  can be formed using any one of the known thin film formation methods, such as thermally assisted evaporation methods, electron beam assisted evaporation methods, chemical vapor deposition methods, plasma enhanced chemical vapor deposition methods, or physical vapor deposition methods. In this case, it is preferred that the thin film resistive layer  18  has a thickness ranging from 100 to 1000 angstroms. 
     Next, photoresist is applied to the thin film resistive layer  18  by a dry film or a spray coating process and then the thin film resistive layer  18  is patterned by photolithography as shown in FIG. 5, so that patterned thin film resistors  18   a ,  18   b ,  18   c , and  18   d  are formed. Then, the patterned thin film resistors  18   a ,  18   b ,  18   c , and  18   d  are trimmed using laser trimming to form kerfs  20   a ,  20   b ,  20   c ,  22   a ,  22   b , and  22   c , in order to control the resistance of each thin film resistor as shown in the cross-sectional view FIG.  6  and its corresponding schematic view FIG.  7 . Thereby, a thin film resistor with desired resistance can be formed. 
     Subsequently, a protecting layer (not shown) is formed over the patterned thin film resistors  18   a ,  18   b ,  18   c , and  18   d , so that the patterned thin film resistors  18   a ,  18   b ,  18   c , and  18   d  are sealed. Any one of the known sealant materials used in the art of thin film resistor manufacturing can be used, for example epoxy sealants, urethane sealants, or silicone sealants. Finally, the insulator substrate  10  is cut along the scribe lines to form discrete thin film resistors. 
     While the present invention has been particularly described, in conjunction with specific preferred embodiments, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the previous description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention. For example, the kerfs  20   a ,  20   b ,  20   c ,  22   a ,  22   b , and  22   c  can be formed using laser trimming of the patterned thin film resistors  18   a ,  18   b ,  18   c , and  18   d ; in addition, the kerfs  20   a ,  20   b ,  20   c ,  22   a ,  22   b , and  22   c  can be simultaneously formed by photolithography along with the patterned thin film resistors  18   a ,  18   b ,  18   c , and  18   d . Thus, at the same time when the thin film resistive layer  18  is patterned by photolithography, the resistance of the thin film resistive layer  18  is adjusted. This can be done by changing the mask pattern.