Abstract:
A system and method is disclosed for using plasma to adjust the resistance of a thin film resistor. In one advantageous embodiment of the invention, the resistance of a thin film resistor is increased to cause the thin film resistor to have a desired higher value of resistance. The thin film resistor is formed having an initial value of resistance that is less than the desired value of resistance. Then the thin film resistor is placed in an oxidizing atmosphere. A surface of the thin film resistor is then oxidized to increase the initial value of resistance to the desired value of resistance. The amount of the increase in resistance may be selected by selecting the temperature of the oxidizing atmosphere.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The invention relates generally to the manufacturing of semiconductor integrated circuits and, more particularly, to the use of plasma to adjust the resistance of thin film resistors. 
   BACKGROUND OF THE INVENTION 
   In the manufacture of semiconductor circuits resistor elements are commonly created using thin film resistors (“TFR”). Thin film resistors are usually made of a metal that is combined with another material to mitigate the resistance of the metal and make the thin film resistor material have a high resistance. One typical example of a thin film resistor material is silicon carbide chrome (“SiCCr”). Another typical example is tantalum nitride (“TaN”). 
   Because a high resistance per unit area is generally desired for a thin film resistor, the thickness of a thin film resistor is usually very thin. For example, a thin film resistor may have a thickness from seventy five Ångstroms (75 Å) to one hundred Ångstroms (100 Å). This thickness range may also be expressed as a range from seven and one half nanometers (7.5 mm) to ten nanometers (10.0 nm). 
   The requirement that a thin film resistor be very thin presents manufacturing problems. During the manufacturing process it may be very difficult to accurately control the thickness of a thin film resistor. The deposition rate of a thin film resistor is relatively high. Any slight variation in the deposition rate will change the thickness of the thin film resistor and affect the value of the resistance of the thin film resistor. Variation in the resistance of the thin film resistors in a semiconductor circuit causes inferior integrated circuit device performance, leading to low yields, increased scrap, and lost market opportunities. 
   Therefore, it would be desirable to have a method to adjust the value of resistance of a thin film resistor after the thin film resistor has been manufactured. One prior art method for adjusting the value of resistance of a thin film resistor employs thermal cycles. However, the use of thermal cycles is not always practical. One problem is that a thermal cycle is not self limiting. It is possible that the thermal cycle method may introduce more variation in the resistance of a thin film resistor than it eliminates. 
   Therefore, there is a need in the art for an efficient system and method for adjusting a value of resistance of a thin film resistor after the thin film resistor has been deposited in a semiconductor manufacturing process. 
   SUMMARY OF THE INVENTION 
   In order to address the deficiencies of the prior art, it is a primary object of the present invention to provide a system and method for adjusting a value of resistance of a thin film resistor. 
   In one advantageous embodiment of the present invention, an initial resistance of a thin film resistor is increased to cause the thin film resistor to have a desired higher value of resistance. A thin film resistor is formed having an initial value of resistance that is less than a desired value of resistance. Then the thin film resistor is placed in an oxidizing atmosphere. A surface of the thin film resistor is then oxidized to increase the initial value of resistance to the desired value of resistance. 
   As the surface of the thin film resistor is oxidized, the oxide layer slows the arrival of new oxidants to the remaining unoxidized thin film resistor material. As the thickness of oxide increases, the oxidation rate slows because of the longer time needed for the oxidizing specie to diffuse through the oxide layer. 
   The system and method of the invention provides more precise control in the manufacture of thin film resistors. For example, a thin film resistor may be deposited with a relatively thick layer. The deposition equipment may be quite capable of precisely controlling the deposition of the thin film resistor for the relatively thick layer. After the thin film resistor has been deposited, the resistance of the thin film resistor will be too low. However, the resistance of the thin film resistor may be increased to its desired value using the oxidation method of the present invention. 
   The amount of the increase in resistance for a thin film resistor may be selected by selecting the temperature of the oxidizing atmosphere. This is because the diffusion rate of the oxidation process is dependent on the temperature at which the oxidation process is carried out. 
   It is an object of the present invention to provide a system and method for adjusting a value of resistance of a thin film resistor. 
   It is also an object of the present invention to provide a system and method for oxidizing a surface of a thin film resistor to increase a value of resistance of the thin film resistor. 
   It is yet another object of the present invention to provide a system and method for oxidizing a surface of a thin film resistor by placing the thin film resistor in a downstream plasma reactor unit. 
   It is still another object of the present invention to provide a system and method for oxidizing a surface of a thin film resistor by using a diffusion limited oxidation process. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
       FIG. 1  illustrates a flow chart showing a semiconductor thin film resistor manufacturing process according to a prior art method; 
       FIG. 2  illustrates a flow chart showing a semiconductor thin film resistor manufacturing process according to an exemplary embodiment of the invention; 
       FIG. 3  illustrates an exemplary thin film resistor to which free oxygen radicals of an oxygen plasma are applied in accordance with the principles of the present invention; 
       FIG. 4  illustrates an exemplary result of applying free oxygen radicals of an oxygen plasma to the thin film resistor shown in  FIG. 3 ; and 
       FIG. 5  illustrates an exemplary graph showing that the value of resistance of a thin film resistor increases linearly with time when the thin film resistor is exposed to the free oxygen radicals of an oxygen plasma. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged thin film resistor manufacturing system. 
     FIG. 1  illustrates a flow chart  100  showing a prior art semiconductor thin film resistor manufacturing process. Initially, a suitable semiconductor wafer is supplied and an insulating layer is deposited on the semiconductor wafer (step  110 ). Then the thin film resistor (TFR) is deposited on the insulating layer (step  120 ). In an optional step, the sheet resistance of the thin film resistor is measured and a thickness of the thin film resistor is derived (step  130 ). Then the thin film resistor is patterned and the manufacture of the thin film resistor module is completed using conventional prior art methods (step  140 ). Then a thermal cycle method is applied to adjust a value of resistance of the thin film resistor (step  150 ). 
   As previously mentioned, the prior art thermal cycle method for adjusting a value of resistance of a thin film resistor is not always practical. One problem is that a thermal cycle is not self limiting. It is possible that the thermal cycle method may introduce more variation in the resistance of the thin film resistor than it eliminates. 
     FIG. 2  illustrates a flow chart  200  showing a semiconductor thin film resistor manufacturing process according to an exemplary embodiment of the invention. Initially, a suitable semiconductor wafer is supplied and an insulating layer is deposited on the semiconductor wafer (step  210 ). Then the thin film resistor (TFR) is deposited on the insulating layer (step  220 ). In an optional step, the sheet resistance of the thin film resistor is measured and a thickness of the thin film resistor is derived (step  230 ). 
   Then the thin film resistor is placed in an oxidizing atmosphere in a downstream oxygen plasma reactor unit (step  240 ). Free oxygen radicals are applied to the surface of the thin film resistor and the surface of the thin film resistor is oxidized (step  250 ). The oxidation of the thin film resistor increases the resistance of the thin film resistor (step  260 ). Because the oxidation process is diffusion limited, the oxidation process stops when the surface of the thin film resistor has been oxidized (step  270 ). Then the thin film resistor is patterned and the manufacture of the thin film resistor module is completed using conventional prior art methods (step  280 ). 
     FIG. 3  illustrates an exemplary thin film resistor  300  to which free oxygen radicals are applied in accordance with the principles of the present invention. The thin film resistor  300  illustrated in  FIG. 3  comprises a thin layer of silicon carbide chrome (SiCCr). The application of the free oxygen radicals to the surface of the thin film resistor  300  is represented by the arrows that point from the chemical symbol for oxygen (O). 
     FIG. 4  illustrates an exemplary thin film resistor  400  that results from applying the free oxygen radicals to the thin film resistor  300  shown in  FIG. 3 . The oxidized top portion of the thin film resistor  400  is designated with reference numeral  410 . Oxidized top portion  410  is made up of the oxidation products of the silicon carbide chrome (SiCCr). Specifically, the oxidized top portion  410  contains, for example, silicon dioxide (SiO 2 ) and chromium oxide (CrO 3 ). In addition, carbon dioxide (CO 2 ) gas evolves from the oxidized top portion  410  of thin film resistor  400 . The evolution of the carbon dioxide (CO 2 ) gas from the surface of the oxidized top portion  410  of thin film resistor  400  is represented by the arrows that point towards the chemical symbol for carbon dioxide (CO 2 ). 
     FIG. 5  illustrates an exemplary graph  500  showing that the value of resistance of a thin film resistor increases linearly with the time that the thin film resistor is exposed to free oxygen radicals of an oxygen plasma ash. The horizontal axis of exemplary graph  500  records the exposure time in seconds of a thin film resistor to an oxygen plasma ash. The horizontal axis is labeled “Ash time (secs)”. The vertical axis of exemplary graph  500  records the change (i.e., increased values) of sheet resistance of the thin film resistor as a function of time. The vertical axis is labeled “Change in Rsh”.  FIG. 5  illustrates that the diffusion process initially increases the resistance of the thin film resistor linearly over time. 
   As previously mentioned, because the oxidation process is diffusion limited, the oxidation process is self limited. At some point in time, the increasing thickness of oxidation compounds (e.g., SiO 2  and CrO 3 ) in top layer  410  of thin film resistor  400  eventually reduce the amount of thin film resistor material (e.g., SiCCr  300 ) that is exposed to the oxygen plasma. Then the increase in resistance of the thin film resistor  400  no longer increases linearly with time. 
   However, the resistance of the thin film resistor  400  may be further increased by increasing the diffusion rate of the oxygen. This may be accomplished by increasing the temperature at which the thin film resistor  400  is exposed to the oxygen plasma. For example, assume that when a thin film resistor  400  is exposed to an oxygen plasma at room temperature (approximately twenty three degrees Centigrade (23° C.) an oxidized top layer  410  is observed to form that has a thickness that is “X Ångstroms.” Then when the thin film resistor  400  is exposed to an oxygen plasma at approximately fifty degrees Centigrade (50° C.) an oxidized top layer  410  is observed to form that has a thickness that is “(X+10) Ångstroms.” The increase in temperature increases the diffusion rate of the oxygen. The increased diffusion rate of the oxygen increased the thickness of the oxidized top layer  410 . By making empirical measurements, it is possible to obtain a relationship between the increases in thin film resistance as a function of the increases in temperature. 
   An advantageous application of the present invention is to provide more precise control in the manufacture of thin film resistors. For example, a thin film resistor may be deposited with a relatively thick layer. The deposition equipment available may be quite capable of precisely controlling the deposition of the thin film resistor for the relatively thick layer. After the thin film resistor has been deposited, the resistance of the thin film resistor will be too low. However, the resistance of the thin film resistor may be increased to its target value using the oxidation method of the present invention. 
   For a numerical example, assume that a thin film resistor having a thickness of one hundred Ångstroms (100 Å) has a sheet resistance of approximately one thousand ohms (1000 Ω) per square unit area. Also assume that the same thin film resistor having a thickness of fifty Ångstroms (50 Å) has a sheet resistance of approximately two thousand ohms (2000 Å) per square unit area. 
   It is considerably more difficult to control the manufacture of the thin film resistor when the thickness of the thin film resistor is fifty Ångstroms (50 Å). However, if one takes the thin film resistor having a thickness of one hundred Ångstroms (100 Å) and oxidizes a top layer of fifty Ångstroms (50 Å) the resulting thin film resistor will also have a resistance of two thousand ohms (2000 Ω) per square unit area. 
   Rather than having a very difficult to control fifty Ångstrom (50 Å) metal deposition, one has a relatively easier to control one hundred Ångstroms (100 Å) metal deposition followed by a self limiting oxidation step. The thicker (100 Å) thin film resistor that is subsequently oxidized has the same resistance (2000 Ω/unit area) as the thinner (50 Å) thin film resistor. The present invention greatly facilitates the efficient manufacture of thin film resistors. 
   Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.