Abstract:
A device and a method of forming the device are disclosed. The device includes a reflector, a first dielectric layer disposed on the reflector, and a thin film resistor disposed on the reflector. The reflector acts as a barrier between the thin film resistor and an underlying dielectric layer which may have a non-uniform thickness. Thus, the thickness control and uniformity of the dielectric layer underlying the reflector does not affect the laser trimming of the thin film resistor. In addition to serving as a barrier, the reflector reflects the trimming laser energy back towards the thin film resistor, thereby improving the efficiency of the laser trimming of the thin film resistor. Furthermore, the thickness of the first dielectric layer situated below the thin film resistor and above the reflector can be easily controlled to substantially optimize the laser trimming efficiency of the thin film resistor.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to semiconductor processing, and in particular, to a device and method which improves the laser trimming of a thin film resistor.  
       BACKGROUND OF THE INVENTION  
       [0002]     Thin film resistors are employed in many integrated circuits. These resistors are used in integrated circuits to implement the desired functionality of circuits, including biasing of active devices, serving as voltage dividers, assisting in impedance matching, etc. They are typically formed by deposition of a resistive material on a dielectric layer, and subsequently patterned to a desired size and shape. Deposition of the resistive material can be performed by any deposition means, such as by sputtering. Often, a thin film resistor is subjected to a heat treatment process (i.e. annealing) to improve its stability and to bring the resistance to a desired value range.  
         [0003]     Although annealing a thin film resistor may alter its resistance to a desired value range, to achieve a precise value for the resistance, laser trimming of the thin film resistor is employed. Laser trimming of a thin film resistor consists of directing a laser beam upon the thin film resistor which causes a change in the thin film material. The change in the thin film material, in turn, causes a corresponding change of its resistance. Accordingly, laser trimming allows precise control of setting the desired resistance for the thin film resistor.  
         [0004]     Typically, it is desirable that laser trimming of thin film resistors be performed in a substantially efficient manner. The efficiency of laser trimming of thin film resistor depends on the thickness, among other factors, of the dielectric situated directly below and supporting the thin film resistor. That is, given a particular thin film resistor material, a wavelength of the laser used in trimming the thin film resistor, and a particular material for the dielectric situated directly below and supporting the thin film resistor, there is a particular thickness of such dielectric material which optimizes the efficiency of laser trimming the thin film resistor.  
         [0005]     However, achieving a desired thickness for such dielectric material may be difficult. Often, thin film resistors are disposed on interlayer dielectrics (ILDs), which in turn, are disposed on irregular topology. Accordingly, the dielectric thickness below various thin film resistors formed on the ILD may vary substantially. In addition, often such dielectric layers are subjected to chemical mechanical polishing (CMP), spin on glass (SOG) etchback, or other planarization techniques. Such techniques typically preclude the ability of accurate thickness control of the dielectric supporting the thin film resistor.  
       SUMMARY OF THE INVENTION  
       [0006]     An aspect of the invention relates to a device comprising a reflector, a first dielectric layer disposed on the reflector, and a thin film resistor disposed on the first dielectric layer. The reflector acts as a barrier between the thin film resistor and an underlying dielectric layer which may have a non-uniform thickness. Thus, the thickness control and uniformity of the dielectric layer underlying the reflector does not affect the laser trimming of the thin film resistor. In addition to serving as a barrier, the reflector reflects the trimming laser energy back towards the thin film resistor, thereby improving the efficiency of the laser trimming of the thin film resistor. Furthermore, the thickness of the first dielectric layer situated below the thin film resistor and above the reflector can be easily controlled to substantially optimize the laser trimming efficiency of the thin film resistor.  
         [0007]     In the exemplary embodiment described herein, the reflector is made of a relatively high melting point material, such as a refractory metal. Examples of reflector materials include tungsten (W), molybdenum (Mo), tantalum (Ta), Rhenium (Re), and/or Niobium (Nb). The first dielectric layer situated below the thin film resistor and above the reflector may be formed of silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), and/or other suitable dielectric materials. The thin film resistor may be formed of chromium silicon (CrSi), nickel chromium (NiCr), tantalum nitride (TaN), and/or other suitable resistive materials.  
         [0008]     In addition, the device may further comprise a second dielectric layer disposed over the thin film resistor. The thickness and other properties of the second dielectric layer may also be chosen so as to substantially optimize the laser trimming efficiency of the thin film resistor. The second dielectric layer may be formed of silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), and/or other suitable dielectric materials. The device may further comprise a metal-insulator-metal (MIM) capacitor in which one of its plates may be formed from the same layer that is used to form the reflector.  
         [0009]     Another aspect of the invention relates to a method of forming such device. Other aspects, features and techniques of the invention will become apparent to one skilled in the relevant art in view of the following detailed description of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  illustrates a side cross-sectional view of an exemplary device having a thin film resistor in accordance with an embodiment of the invention;  
         [0011]     FIGS.  2 A-E illustrate side cross-sectional views of an exemplary device at various stages pursuant to a method of forming such device in accordance with another embodiment of the invention;  
         [0012]      FIG. 3  illustrates a side cross-sectional view of another exemplary device having a thin film resistor in accordance with another embodiment of the invention; and  
         [0013]     FIGS.  4 A-F illustrate side cross-sectional views of another exemplary device at various stages pursuant to another method of forming such device in accordance with another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  illustrates a side cross-sectional view of an exemplary device  100  having a thin film resistor in accordance with an embodiment of the invention. The device  100  comprises a first dielectric layer  102 , a reflective layer (reflector)  104  situated over the dielectric layer  102 , a second dielectric layer  106  situated over the reflective layer  104  and the first dielectric layer  102 , a thin film resistor  108  situated over the second dielectric layer  106 , and a third dielectric layer  110  situated over the thin film resistor  108 .  
         [0015]     The first dielectric layer  102  is typically an interlayer dielectric (ILD) or other dielectric layer. Typically, the thickness of the first dielectric layer  102  is difficult to control precisely. Therefore, for the sake of laser trimming efficiency, it would not be desirable to dispose the thin film resistor on such layer.  
         [0016]     The reflective layer  104  should be substantially reflective to the laser energy used in laser trimming the thin film resistor  108 . The reason being is that to improve the efficiency of laser trimming the thin film resistor  108 , it is desirable for the laser energy that passes through the thin film resistor  108  to reflect off of the reflective layer  104  and to propagate back to the thin film resistor  108 . In the exemplary embodiment, the reflective layer  104  may comprise a relatively high melting point refractory material to prevent melting of the reflective layer  104  during laser trimming of the thin film resistor  108 . For instance, the reflective layer  104  may comprise a refractory metal, such as tungsten (W), molybdenum (Mo), tantalum (Ta), Rhenium (Re), Niobium (Nb), etc.  
         [0017]     The deposition of the second dielectric layer  106  should be precisely controlled such that a particular thickness is achieved for the second dielectric layer  106  which substantially optimizes the efficiency of laser trimming the thin film resistor  108 . Such thickness typically depends on the wavelength of the laser energy and the intrinsic properties of the second dielectric layer  106 , such as its index of refraction and extinction coefficient. As illustrated in  FIG. 1 , the optimal thickness of the second dielectric layer  106  results in the incident laser energy and the reflective laser energy below the thin film resistor  108  constructively combining at the thin film resistor  108  to produce a more efficient trimming of the resistor. In the exemplary embodiment, the second dielectric layer  106  may comprise silicon dioxide (SiO 2 ) and/or silicon nitride (Si 3 N 4 ).  
         [0018]     The thin film resistor  108  may be formed of suitable thin film resistive material. For instance, the thin film resistor  108  may be comprised of chromium silicon (CrSi), nickel chromium (NiCr), tantalum nitride (TaN), and/or others.  
         [0019]     The deposition of the third dielectric layer  110  should be precisely controlled such that a particular thickness is achieved for the third dielectric layer  110  which substantially optimizes the efficiency of the laser trimming of the thin film resistor  108 . Such thickness typically depends on the wavelength of the laser energy and the intrinsic properties of the third dielectric layer  110 , such as its index of refraction and extinction coefficient. As illustrated in  FIG. 1 , the optimal thickness of the third dielectric layer  110  results in the incident laser energy and the reflective laser energy above the thin film resistor  108  constructively combining at the thin film resistor  108  to produce a more efficient trimming of the resistor. In the exemplary embodiment, the third dielectric layer  110  may comprise silicon dioxide (SiO 2 ) and/or silicon nitride (Si 3 N 4 ).  
         [0020]     An advantage of the device  100  over prior art devices is that the thickness of the first dielectric layer  102  does not affect the laser trimming of the thin film resistor  108 . This is because the reflective layer  104  acts as a barrier, for laser trimming purposes, between the first dielectric layer  102  and the thin film resistor  108 . Thus, the thickness control of the first dielectric layer  102  need not be precise for resistor laser trimming purposes. A method of forming such a device is discussed below.  
         [0021]      FIG. 2A  illustrates a side cross-sectional view of an exemplary device  200  at a particular stage pursuant to a method of forming such device in accordance with another embodiment of the invention. At this stage, the device  200  comprises a first dielectric layer  202 , a reflective layer  204  disposed over the first dielectric layer  202 , and a mask layer  206  disposed over the reflective layer  204 . As previously discussed with reference to device  100 , the first dielectric layer  202  may be an interlayer dielectric (ILD) or other dielectric layer, which need not have a precise thickness for resistor laser trimming purposes. The reflective layer  204  may be comprised of a relatively high melting point material, such as a refractory metal. The mask layer  206  may be of a suitable material (e.g. photoresist) to serve as a mask in the etching of the reflective layer  204 .  
         [0022]      FIG. 2B  illustrates a side cross-sectional view of an exemplary device  200  at a subsequent stage pursuant to a method of forming such device in accordance with another embodiment of the invention. According to the method, the mask layer  206  is patterned and developed to form a mask  206 ′ which is used to define the to-be formed reflector. Then the device  200  is subjected to an etching process to substantially remove all of the reflective layer  204  except that portion underlying the mask  206 ′. This etching process forms a reflector  204 ′ used to improve the efficiency in laser trimming of a thin film resistor.  
         [0023]      FIG. 2C  illustrates a side cross-sectional view of an exemplary device  200  at a subsequent stage pursuant to a method of forming such device in accordance with another embodiment of the invention. According to the method, the mask  206 ′ is removed. Then, a second dielectric layer  208  is formed over the reflector  204 ′ and the first dielectric layer  202 , a thin film resistive layer  210  is formed over the second dielectric layer  208 , and a mask layer  212  is formed over the thin film resistive layer  210 . As previously discussed with reference to device  100 , the second dielectric layer  208  may be formed of suitable dielectric material, such as SiO 2  and/or Si 3 N 4 . The thin film resistive layer  210  may be formed of suitable resistive material, such as CrSi, NiCr, and/or TaN. The mask layer  212  may be of a suitable material (e.g. photoresist) to serve as a mask in the etching of the thin film resistive layer  210 .  
         [0024]      FIG. 2D  illustrates a side cross-sectional view of an exemplary device  200  at a subsequent stage pursuant to a method of forming such device in accordance with another embodiment of the invention. According to the method, the mask layer  212  is patterned and developed to form a mask  212 ′ which is used to define the to-be formed thin film resistor. Then the device  200  is subjected to an etching process to substantially remove all of the thin film resistive layer  210  except that portion underlying the mask  212 ′. This etching process forms a thin film resistor  210 ′ (see  FIG. 2E ). Subsequent to the formation of the thin film resistor  212 ′, a third dielectric layer is formed over the thin film resistor, such as dielectric layer  110  shown in  FIG. 1 .  
         [0025]      FIG. 3  illustrates a side cross-sectional view of another exemplary device  300  having a thin film resistor in accordance with another embodiment of the invention. The device  300  is similar to that of device  100 . Accordingly, elements common to both devices  100  and  300  have the same reference numbers, except that the most significant digit is “1” in the case of device  100  and “3” in the case of device  300 . Thus, the device  300  comprises a first dielectric layer  302 , a reflective layer (reflector)  304  situated over the first dielectric layer  302 , a second dielectric layer  306  situated over the reflective layer  304 , a thin film resistor  308  situated over the second dielectric layer  306 , and a third dielectric layer  310  situated over the thin film resistor  308 .  
         [0026]     The device  300  differs from device  200  in that device  300  further comprises a metal-insulator-metal (MIM) capacitor  312 . In this example, the MIM capacitor  312  comprises a lower conductive plate  314 , a dielectric  316 , and an upper conductive plate  318 . The upper conductive plate  318  is formed from the same layer used to form the reflector  304 . This illustrates that other features of the device may be formed using the reflective layer  304 . The dielectric layer  316  used in the MIM capacitor  312  may also be formed between the first dielectric layer  302  and the reflective layer  304 . An example of a method of forming such a device is discussed below.  
         [0027]      FIG. 4A  illustrates a side cross-sectional view of an exemplary device  400  at a particular stage pursuant to a method of forming such device in accordance with another embodiment of the invention. At this stage, the device  400  comprises a first dielectric layer  402 , an electrically-conductive layer  404  disposed over the first dielectric layer  402 , and a mask layer  406  disposed over the electrically-conductive layer  404 . As previously discussed, the first dielectric layer  402  may be an interlayer dielectric (ILD) or other dielectric layer, which need not have a precise thickness for resistor laser trimming purposes. The electrically-conductive layer  404  may be comprised of a metallization layer and/or a doped polycrystalline (“polysilicon”) layer. The mask layer  406  may be of a suitable material (e.g. photoresist) to serve as a mask in the etching of the electrically-conductive layer  404 .  
         [0028]      FIG. 4B  illustrates a side cross-sectional view of an exemplary device  400  at a subsequent stage pursuant to a method of forming such device in accordance with another embodiment of the invention. According to the method, the mask layer  406  is patterned and developed to form a mask  406 ′ which is used to define the to-be formed lower plate of the MIM capacitor. Then the device  400  is subjected to an etching process to substantially remove all of the electrically-conductive layer  404  except that portion underlying the mask  406 ′. This etching process forms a lower plate  404 ′ of the to-be formed MIM capacitor.  
         [0029]      FIG. 4C  illustrates a side cross-sectional view of an exemplary device  400  at a subsequent stage pursuant to a method of forming such device in accordance with another embodiment of the invention. According to the method, the mask  406 ′ is removed. Then, a second dielectric layer  408  is formed over the lower plate  404 ′ of the to-be formed MIM capacitor and the first dielectric layer  402 . Then, an electrically-conductive reflective layer  410  is formed over the second dielectric layer  408 , and a mask layer  412  is formed over the electrically-conductive reflective layer  410 . As previously discussed, the second dielectric layer  208  serves as the dielectric for the to-be formed MIM capacitor, and may be formed of a suitable dielectric material, such as SiO 2  and/or Si 3 N 4 . The electrically-conductive reflective layer  410  may be comprised of a relatively high melting point material, such as a refractory metal. The mask layer  412  may be of a suitable material (e.g. photoresist) to serve as a mask in the etching of the electrically-conductive reflective layer  410 .  
         [0030]      FIG. 4D  illustrates a side cross-sectional view of an exemplary device  400  at a subsequent stage pursuant to a method of forming such device in accordance with another embodiment of the invention. According to the method, the mask layer  412  is patterned and developed to form a mask  412 ′-a that is used to define the to-be formed upper plate of the to-be formed MIM capacitor, and a mask  412 ′-b that is used to define the to-be formed reflector. Then the device  400  is subjected to an etching process to substantially remove all of the reflective layer  410  except those portions underlying the respective masks  412 ′a-b. This etching process forms the upper plate  410 ′-a of the MIM capacitor and the reflector  410 ′-b.  
         [0031]      FIG. 4E  illustrates a side cross-sectional view of an exemplary device  400  at a subsequent stage pursuant to a method of forming such device in accordance with another embodiment of the invention. According to the method, a third dielectric layer  414  is formed over the upper plate  410 ′-a of the MIM capacitor, the second dielectric layer  408 , and the reflector  410 ′-b. Then, a thin film resistive layer  416  is formed over the third dielectric layer  414 , and a mask  418  is formed over the thin film resistive layer  416 . As previously discussed, the third dielectric layer  414  may be formed of a suitable dielectric material; such as SiO 2  and/or Si 3 N 4 . Also, the thickness of the third dielectric layer  414  should be such as to optimize the laser trimming of the to-be formed thin film resistor. The thin film resistive layer  416  may be formed of suitable resistive material, such as CrSi, NiCr, and/or TaN. The mask layer  418  may be of a suitable material (e.g. photoresist) that serves as a mask in the etching of the thin film resistive layer  416 .  
         [0032]      FIG. 4F  illustrates a side cross-sectional view of an exemplary device  400  at a subsequent stage pursuant to a method of forming such device in accordance with another embodiment of the invention. According to the method, the mask layer  418  is patterned and developed to form a mask  418 ′ which is used to define the to-be formed thin film resistor. Then the device  400  is subjected to an etching process to substantially remove all of the thin film resistive layer  416  except that portion underlying the mask  418 ′. This etching process forms thin film resistor  308  (See  FIG. 3 ). Subsequent to the formation of the thin film resistor  308 , a third dielectric layer is formed over the thin film resistor, such as dielectric layer  310  shown in  FIG. 3 .  
         [0033]     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.