Abstract:
A thin film resistor is formed to have very accurately defined dimensions which, in turn, allow the resistive value of the resistor to be very accurately defined. The resistor is formed on spaced-apart conductive pads which, in turn, are electrically connected to conductive plugs that are spaced apart from the resistor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to thin film resistors and, more particularly, to a thin film resistor and method of forming the resistor on spaced-apart conductive pads. 
   2. Description of the Related Art 
   A thin film resistor is a structure that is formed from a conducting resistive material. As with conventionally-formed discrete resistors, thin film resistors are formed to provide a predefined resistance to the flow of current through the semiconductor structure. 
     FIGS. 1A-1B  to  5 A- 5 B show a series of views that illustrate a prior-art method  100  of forming a thin film resistor.  FIGS. 1A-5A  show a series of plan views, while  FIGS. 1B-5B  show a series of corresponding cross-sectional views. As shown in  FIGS. 1A-1B , method  100  begins with a conventionally-formed layer of insulation material  110 , and continues with the deposition of a thin layer of resistor material  112 , such as a layer of silicon carbide chrome (SiCCr) or nickel chrome (NiCr), on insulation layer  110 . 
   After resistor material  112  has been deposited, a mask  114  is formed and patterned on resistor material  112 . Following this, as shown in  FIGS. 2A-2B , the exposed areas of resistor material  112  are etched to form a thin-film resistor  116  from resistor material  112 . Once the etch has been completed, mask  114  is removed. 
   Next, as shown in  FIGS. 3A-3B , a first layer of conductive material  120 , such as titanium tungsten (TiW), is formed on insulation layer  110  and resistor  116 . After this, a second layer of conductive material  122 , such as aluminum, is formed on the first layer of conductive material  120 . 
   Once the second layer of conductive material  122  has been formed, a mask  124  is formed and patterned on the second layer of conductive material  122 . Following this, as shown in  FIGS. 4A-4B , the exposed areas of the second layer of conductive material  122  are anisotropically etched, followed by the anisotropic etching of a portion of the exposed areas of the first layer of conductive material  120  to form an opening  126 . 
   Since the first layer of conductive material  120  is partially removed with an anisotropic (dry) etch, the first layer of conductive material  120  must be sufficiently thick to ensure that the anisotropic etch does not etch through the first layer of conductive material  120  and erode or remove any portion of thin-film resistor  116  that lies underneath. 
   After the anisotropic etch has been completed, the exposed areas of the first layer of conductive material  120  are isotropically (wet) etched as shown in  FIGS. 5A-5B  with an etchant that has a high selectivity to the material of resistor  116  until the first layer of conductive material  120  has been removed from the top surface of resistor  116 . Following this, mask  124  is removed. 
   One problem with method  100  is that the first layer of conductive material  120 , which has to be sufficiently thick to avoid damage to thin-film resistor  116 , must be wet etched for a relatively long period of time (over etched) even though it has been partially etched during the anisotropic etch to ensure that the first layer of conductive material  120  has been completely removed. 
   If the first layer of conductive material  120  is not completely removed, stringers  128  can remain which, in turn, can short out the resistor. Stringers  128  are tiny strips of the first layer of conductive material  120  which can remain after the first layer of conductive material  120  has been removed from the top surface of resistor  116 . 
   However, the longer the first layer of conductive material  120  is exposed to the isotropic etchant to ensure the removal of stringers  128 , the greater the length L 1  (the width of the opening shown in  FIG. 5B ). The length L 1  defines the length of resistor  116  which, in turn, defines (in part) the resistance provided by resistor  116 . As a result, it becomes difficult to control the resistance provided by resistor  116 . 
   Thus, there is a need for a thin film resistor and method of forming the resistor that reduces variations in the length of the resistor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-1B  to  5 A- 5 B are a series of views illustrating a prior-art method  100  of forming a thin film resistor.  FIGS. 1A-5A  are a series of plan views, while  FIGS. 1B-5B  are a series of cross-sectional views. 
       FIGS. 6A-6B  to  17 A- 17 B are a series of views illustrating an example of a method  200  of forming a thin film resistor in accordance with the present invention.  FIGS. 6A-17A  are a series of plan views, while  FIGS. 6B-17B  are a series of cross-sectional views. 
       FIGS. 18A-18B  to  23 A- 23 B are a series of views illustrating an example of a method  300  of alternately forming a thin film resistor in accordance with the present invention.  FIGS. 18A-23A  are a series of plan views, while  FIGS. 18B-23B  are a series of cross-sectional views. 
       FIG. 24  is a plan view similar to  FIG. 9A  illustrating resistor  234  in accordance with an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 6A-6B  to  17 A- 17 B show a series of views that illustrate an example of a method  200  of forming a thin film resistor in accordance with the present invention.  FIGS. 6A-17A  show a series of plan views, while  FIGS. 6B-17B  show a series of cross-sectional views. As described in greater detail below, the present invention forms a thin film resistor on a pair of spaced-apart conductive pads which, in turn, allows electrical contacts to be made to the conductive pads rather than directly to the resistor. 
   As shown in the  FIGS. 6A-6B  example, method  200  utilizes a semiconductor wafer  210  which has been conventionally processed to form layer of insulation material  212  on semiconductor wafer  210 , a number of metal-1 traces  214  that have been formed on insulation layer  212 , and a layer of isolation material  216  that has been formed on insulation layer  212  and the metal-1 traces  214 . Isolation layer  216 , which has been planarized, can be formed to have a thickness of, for example, 4500 Å. 
   As further shown in  FIGS. 6A-6B , method  200  begins by forming and patterning a mask  220  on the top surface of isolation layer  216 . Following this, the exposed regions of isolation layer  216  are etched to remove, for example, 1500 Å of isolation layer  216  to form a pair of spaced-apart openings  222 . Each opening  222  can have, for example, a 1 μM diameter. Resist layer  220  is then stripped (e.g., using conventional ashing procedures), and the top surface of isolation layer  216  is cleaned (e.g., using conventional solvents and procedures). 
   Next, as shown in  FIGS. 7A-7B , a metallic layer  224  is formed on the top surface of isolation layer  216  to fill up the openings  222 . Metallic layer  224  can be formed from, for example, 50 Å of titanium followed by 1600 Å of titanium nitride. The titanium layer is used to improve adhesion, and can be omitted if the adhesion of the titanium nitride layer is satisfactory. The titanium and titanium nitride layers can be, for example, sputter deposited. In addition, metallic layer  224  can also include an overlying layer of tungsten. The tungsten layer can be deposited by, for example, chemical vapor deposition. 
   After this, as shown in  FIGS. 8A-8B , metallic layer  224  is removed from the top surface of isolation layer  216 , along with a portion, such as 500 Å, of the top surface of isolation layer  216  to form a pair of spaced-apart landing pads  226 . For example, metallic layer  224  and the portion of the top surface of isolation layer  216  can be removed using conventional chemical mechanical polishing processes. In addition, metallic layer  224  on isolation layer  216  could also be removed by other means than chemical mechanical polishing, for example plasma etching as has been applied to tungsten plugs and polysilicon trench fill. 
   Thus, in the above example, following the chemical mechanical polishing, the landing pads  226  have 50 Å of titanium and 1100 Å of titanium nitride. (A sputter clean can optionally follow the chemical mechanical polishing to smooth the surface and promote adhesion of the following resistor layer. In this preferred embodiment, the sputter clean is targeted at 50 Å removal.) 
   As shown in  FIGS. 9A-9B , a layer of resistor material  230 , such as silicon carbide chrome or nickel chrome, is then formed on isolation layer  216  and the landing pads  226  to have a thickness of, for example, 75 Å-100 Å, and a resistance of, for example, 1400 ohms/sq, with a 1000 ohms/sq target at the end of fabrication following multiple thermal cycles. 
   Resistor layer  230  can be formed using, for example, sputter deposition with a low energy power supply at a wafer temperature of 40° C. In addition, the layer of resistor material  230  could be formed by other methods, including but not limited to reactive sputtering, co-sputtering, chemical vapor deposition, or sputtering followed by rapid thermal processing. 
   Next, a mask  232  is formed and patterned on resistor layer  230  to protect the portion of resistor layer  230  that lies between the landing pads  226 , and over an inner region of each of the landing pads  226 . The mask grade and photo process preferably accommodate the formation of matching side-by-side resistors with a variation of no more than 0.1% (3 sigma). 
   Following this, as shown in  FIGS. 10A-10B , the exposed regions of resistor layer  230  are etched to form a resistor  234 . Resistor  234  can be, for example, 2-4 μM wide by 20-40 μM long. Resistor layer  230  can be removed using, for example, a plasma etch or a sputter etch. The etch preferably has a reasonable selectivity to titanium nitride and oxide, and removes no more than about 200 Å of the titanium nitride layer from the landing pads  226  when removing 75 Å of resistor layer  230 . As noted above, the landing pads  226  can also include a top layer of tungsten if the titanium nitride selectivity is very poor. 
   Once resistor  234  has been formed, mask  232  is then stripped using, for example, a conventional solvent strip or a nitrogen and hydrogen (N 2 +H 2 ) gas combination. Mask  232  should not be ashed in oxygen (O 2 ) to prevent damage when a silicon carbide chrome resistor is used. 
   As shown in  FIGS. 11A-11B , after mask  232  has been removed, a layer of isolation material  236  is formed on isolation layer  216 , the landing pads  226 , and resistor  234 . For example, isolation layer  236  can be formed by depositing plasma oxide (SiH 4 ) to a thickness of 2500 Å. As a result, the combined thickness of isolation layers  216  and  236  is approximately 6500 Å. After this, a mask  238  is formed and patterned on isolation layer  236 . 
   Next, as shown in  FIGS. 12A-12B , isolation layer  236  and the underlying isolation layer  216  are etched to form via openings  240  that expose the top surfaces of the metal-1 traces  214 . Mask  238  is then removed. As shown in  FIGS. 13A-13B , a via liner  242 , such as a layer of titanium followed by a layer of titanium nitride, is next formed on isolation layer  236  and in via openings  240 , followed by the formation of a layer of tungsten  244  on via liner  242  to fill up via openings  240 . In addition, via openings  240  can be filled by other methods, including but not limited to hot aluminum deposition. 
   After this, as shown in  FIGS. 14A-14B , tungsten layer  244  and via liner  242  are removed from the top surface of isolation layer  236 , along with a portion, such as 500 Å, of the top surface of isolation layer  236  to form conductive plugs  246 . For example, tungsten layer  244 , via liner  242 , and the portion of the top surface of isolation layer  236  can be removed using conventional chemical mechanical polishing processes. In addition, tungsten layer  244 , via liner  242 , and the portion of the top surface of isolation layer  236  can also be removed by other means that chemical mechanical polishing, for example plasma etching as has been applied to tungsten plugs and polysilicon trench fill. Thus, in the above example, following the chemical mechanical polishing, approximately 2000 Å of isolation layer  236  remain over resistor  234 . 
   Next, as shown in  FIGS. 15A-15B , a mask  250  is formed and patterned on isolation layer  236  and the conductive plugs  246 . Isolation layer  236  is then etched to form a pair of resistor openings  252  to expose the top surfaces of the landing pads  226 . The resistor openings  252  can be, for example, approximately 0.2 μM deep and 1.0 μM wide. Following this, mask  250  is removed. 
   As shown in  FIGS. 16A-16B , a metal-2 layer  254  is then formed over isolation layer  236  to fill up resistor openings  252 . Once metal-2 layer  254  has been formed, a mask  256  is formed and patterned on metal-2 layer  254 . After this, as shown in  FIGS. 17A-17B , the exposed regions of metal-2 layer are removed from the top surface of isolation layer  236  to form metal-2 traces  260  that are connected to the conductive plugs  246 , and metal-2 traces  262  that are connected to the landing pads  226 . Following this, mask  256  is removed. The process then continues with conventional back end processing steps. 
     FIGS. 18A-18B  to  23 A- 23 B show a series of views that illustrate an example of a method  300  of alternately forming a thin film resistor in accordance with the present invention.  FIGS. 18A-23A  show a series of plan views, while  FIGS. 18B-23B  show a series of cross-sectional views. Method  300  is similar to method  200  and, as a result, utilizes the same reference numerals to designate the elements and structures which are common to both methods. 
   As shown in  FIGS. 18A-18B , method  300  is the same as method  200  up through the formation of isolation layer  236 , except that the titanium nitride layer of the metallic layer  224  is formed to be, for example, 2000 Å thick as opposed to 1500 Å thick as disclosed in method  200 . Following this, rather than forming mask  238 , a mask  310  is formed and patterned on isolation layer  236 . 
   Next, as shown in  FIGS. 19A-19B , isolation layer  236  and the underlying isolation layer  216  are etched to form via openings  312  that expose the top surfaces of the metal-1 traces  214 . The etch also forms resistor openings  314  in isolation layer  236  that expose the top surfaces of the landing pads  226 . Mask  310  is then removed. 
   As shown in  FIGS. 20A-20B , a liner  316 , such as a layer of titanium followed by a layer of titanium nitride, is next formed over isolation layer  236  and in the via openings  312  and resistor openings  314 , followed by the formation of a layer of tungsten  320  on liner layer  316  to fill up the via openings  312  and resistor openings  314 . In addition, via openings  312  and resistor openings  314  can be filled by other methods, including but not limited to hot aluminum deposition. 
   After this, as shown in  FIGS. 21A-21B , tungsten layer  320 , and liner layer  316  are removed from the top surface of isolation layer  236 , along with a portion, such as 500 Å, of the top surface of isolation layer  236  to form conductive plugs  322  that are connected to the metal-1 traces  214 , and resistor plugs  324  that are connected to the landing pads  226 . 
   For example, tungsten layer  320 , liner layer  316 , and the portion of the top surface of isolation layer  236  can be removed using conventional chemical mechanical polishing processes. In addition, tungsten layer  320 , liner layer  316 , and the portion of the top surface of isolation layer  236  can also be removed by other means that chemical mechanical polishing, for example plasma etching as has been applied to tungsten plugs and polysilicon trench fill. 
   Next, as shown in  FIGS. 22A-22B , a metal-2 layer  330  is then formed over isolation layer  236 , the conductive plugs  322 , and the resistor plugs  324 . Once metal-2 layer  330  has been formed, a mask  332  is formed and patterned on metal-2 layer  330 . After this, as shown in  FIGS. 23A-23B , the exposed regions of metal-2 layer are removed from the top surface of isolation layer  236  to form metal-2 traces  334  that are connected to the resistor plugs  324  and metal-2 traces  336  that are connected to the conductive plugs  322 . Following this, mask  332  is removed. The process then continues with conventional back end processing steps. 
   One of the advantages of the present invention is that the present invention eliminates the need to wet etch the overlying electrical contact, such as conductive layer  120  shown in  FIG. 5B , to ensure the removal of stringers. Thus, the variation in resistor length that results from the wet overetch to remove stringers is eliminated. As a result, the method of the present invention provides a process of forming resistors with highly accurate and matched dimensions. 
   In addition, as shown in  FIGS. 17B and 23B , no metal-1 trace lies above or below resistor  234 . This is to ensure that there are no external influences present for critical resistor matching applications. On the other hand, for non-critical applications, metal traces can lie above and/or below resistor  234 . 
   It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. For example, although resistor  234  is illustrated as formed below the metal-2 layer, resistor  234  can be formed below any metal layer. 
   In addition, although the present invention has been disclosed with resistor  234  extending from one conductive landing pad  226  to another conductive landing pad  226  via a straight line, resistor  234  can alternately extend from one conductive landing pad  226  to another conductive landing pad  226  via any path, such as via a serpentine path. 
     FIG. 24  shows a plan view similar to  FIG. 9A  that illustrates resistor  234  in accordance with an alternate embodiment of the present invention. As shown in  FIG. 24 , resistor  234  extends from one conductive landing pad  226  to another conductive landing pad  226  via an S-shaped path. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.