Abstract:
A method of evaluating laser trimming of a semiconductor device having a thin film resistor is disclosed. The method includes the steps of providing the thin film resistor and laser trimming the thin film resistor by creating a first trim cut. The first trim cut bisects the thin film resistor such that the thin film resistor is divided into a first portion and a second portion. Also, the method involves measuring the insulation resistance of the thin film resistor. In addition, the method involves evaluating the trim cut based on the measured insulation resistance.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2005-211811 filed on Jul. 21, 2005, the content of which is incorporated herein by reference.  
         [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to laser trimming, and more particularly relates to a semiconductor device having a trim cut and a method of evaluating the laser trimming of the semiconductor device.  
         [0004]     2. Background of the Invention  
         [0005]     Laser trimming is used for trimming a thin film resistor (see U.S. Pat. No. 5,081,439 and Japanese Patent No. 2,618,139 for example). Specifically, as shown in  FIGS. 10A and 10B , a thin film resistor  102  is formed on a top surface of a silicon substrate  100  of an IC chip. An insulation film  101  is interposed between the thin film resistor  102  and the silicon substrate  100 . Also, electrodes  103 ,  104  are disposed at opposite ends of the thin film resistor  102 . Trimming is performed by irradiating the thin film resistor  102  with a laser beam to form a trim cut  105 . In the embodiment shown, for instance, an L-shaped trim cut  105  is formed.  
         [0006]     It is known that the quality of trim cuts created during laser trimming can vary. For instance, the cutting quality of a linearly polarized laser beam may be adequate in one linear direction, but uncut strips may remain when trimming in another linear direction. Also, trimming at an inadequate laser intensity (i.e., a shortage of the laser beam energy) can cause defects to be created in the resistor. Defects can also be created due to improper focusing of the laser beam and/or due to a relatively small trimming window (i.e., a relatively small width of trimming energy over which a favorable cut can be achieved).  
         [0007]     The resistor can be visually examined to detect defects; however, a visual examination may not reveal certain defects. In addition, the resistor can be electrically evaluated for detecting defects. However, an electrical evaluation may not reveal certain defects, especially if the trim cut is relatively long. This is because the electrical characteristics of the resistor are more likely to fall within acceptable ranges as the length of the trim cut increases.  
         [0008]     More specifically, as shown in  FIG. 11A , a thin film resistor  102  has been trimmed using a linearly polarized laser beam. The trim cut  105  includes a first trim cut  105   a,  which extends in the Y-direction, and a second trim cut  105   b,  which extends in the X-direction. As shown, the cutting ability of the linearly polarized laser beam is reduced when trimming in the X-direction as compared to trimming in the Y-direction. In other words, uncut parts remain in the second trim cut  105   b , and the uncut parts could be detected with a visual examination. However, uncut parts in the first trim cut  105   a,  if any, are not obviously present, and such defects would be difficult to detect with a visual examination.  
         [0009]      FIG. 11B  also shows an example of trimming using a linearly polarized laser beam, in which the trim cut  105  is formed in the Y-direction only. In this case, uncut parts, if any, are not obviously present in the trim cut  105  because it extends in the Y-direction only. Thus, defects in the trim cut  105  of  FIG. 11B  would be difficult to detect with a visual examination.  
         [0010]      FIG. 11C  shows an example of trim cuts  105   a,    105   b  disposed in an L-shape and formed using a circularly polarized laser beam. As shown, uncut parts, if any, are not obviously present in the trim cuts  105   a,    105   b  because the circularly polarized laser beam was used. As such, any defects in the trim cuts  105   a,    105   b  would be difficult to detect with a visual examination.  
       SUMMARY OF THE INVENTION  
       [0011]     The present disclosure relates to a method of evaluating laser trimming of a semiconductor device having a thin film resistor. The method includes the steps of providing the thin film resistor and laser trimming the thin film resistor by creating a first trim cut. The first trim cut bisects the thin film resistor such that the thin film resistor is divided into a first portion and a second portion. Also, the method involves measuring the insulation resistance of the thin film resistor. In addition, the method involves evaluating the trim cut based on the measured insulation resistance.  
         [0012]     In another aspect, the present disclosure relates to a method of determining an operative laser intensity for laser trimming. The method includes the step of laser trimming a first thin film resistor by creating a first trim cut at a laser intensity of a predetermined value. The first trim cut bisects the first thin film resistor. The method also includes measuring insulation resistance of the first thin film resistor. In addition, the method includes the step of determining the operative laser intensity based on the measured insulation resistance of the first thin film resistor. Furthermore, the method includes laser trimming a second thin film resistor at the operative laser intensity.  
         [0013]     In still another aspect, the present disclosure relates to a semiconductor device that includes a first thin film resistor and a first pad operatively coupled to the first thin film resistor. The semiconductor device further includes a second pad operatively coupled to the first thin film resistor. The semiconductor device also includes a first trim cut that bisects the first thin film resistor between the first pad and the second pad. Furthermore, the semiconductor device includes a measuring device in electrical communication with the first pad and the second pad. The measuring device is operable for measuring an insulation resistance of the first thin film resistor so as to evaluate the first trim cut.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a plan view of a semiconductor device;  
         [0015]      FIGS. 2A and 2B  are plan views of resistors included on the wafer of  FIG. 1 ;  
         [0016]      FIG. 3  is a longitudinal sectional view taken along the line III-III of  FIG. 2B ;  
         [0017]      FIG. 4  is a plan view of a thin film resistor that includes a first trim cut;  
         [0018]      FIG. 5  is a plan view of the thin film resistor of  FIG. 4  that includes additional trim cuts;  
         [0019]      FIG. 6  is a plan view of another exemplary thin film resistor;  
         [0020]      FIGS. 7A and 7B  are plan views of thin film resistors that include trim cuts extending in a variety of directions;  
         [0021]      FIG. 8  is a plan view of a thin film resistor with a trim cut that extends in a direction corresponding to one of the trim cuts of  FIG. 7A ;  
         [0022]      FIG. 9  is a plan view of a thin film resistor with a trim cut that extends in a direction corresponding to a trim cut of  FIG. 7B ;  
         [0023]      FIG. 10A  is a plan view of a thin film resistor for explaining the prior art;  
         [0024]      FIG. 10B  is a longitudinal sectional view taken along the line XB-XB of  FIG. 10A ; and  
         [0025]      FIGS. 11A, 11B , and  11 C are plan views of thin film resistors for explaining the prior art.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     One embodiment of the invention will now be described with reference to the drawings.  FIG. 1  shows a wafer  1  that includes a plurality of first thin film resistors  2  and a plurality of second thin film resistors  18 . In one embodiment, the first and second resistors  2 ,  18  are metal thin film resistors. It will be-appreciated that the wafer  1  could include any number of first and second resistors  2 ,  18 . It will also be appreciated that the wafer  1  could include only the first resistors  2 . Furthermore, it will be appreciated that the first and second resistors  2 ,  18  could be included in any suitable pattern on the wafer  1 .  
         [0027]     As will be explained in greater detail below, one or more of the first resistors  2  is laser trimmed and then evaluated in a manner to be described. By evaluating the trimmed first resistor  2 , appropriate parameters can be determined for the laser trimming of the second resistors  18 . In one embodiment, the first resistors  2  are used only for evaluation of the laser trimming process, and the second resistors  18  are used as electrically resistive components for circuits in the wafer  1 .  
         [0028]      FIGS. 2A and 2B  show a first thin film resistor  2  and a second thin film resistor  18 . As will be explained, the appropriate laser trimming parameters for trimming the second resistor of  FIG. 2A  is discovered by laser trimming the first resistor  2  of  FIG. 2B  and then evaluating the laser trimming of the first resistor  2  of  FIG. 2B .  
         [0029]     In the embodiment shown, the first resistor  2  is rectangular shaped so as to define a first edge  3   a , a second edge  3   b , a third edge  3   c , and a fourth edge  3   d . In one embodiment, the shape of the first resistor  2  corresponds to that of the second resistor  18 . However, it will be appreciated that the first and second resistors  2 ,  18  can have any suitable shape.  
         [0030]     As shown in  FIG. 3 , the first resistor  2  is disposed on a silicon substrate  10  of the wafer  1  with an insulation film  11  interposed between the first resistor  2  and the silicon substrate  10 . The wafer  1  (i.e., the silicon substrate  10 ) is chucked on an X-Y table  28 , and the wafer  1  is moved as a result of horizontal movement of the X-Y table  28  (i.e., back and forth and to the left and right).  
         [0031]     A laser apparatus  20  is provided above the X-Y table  28 . The laser apparatus  20  includes a laser oscillator  21  and a linear polarizer  22 . In one embodiment, the laser apparatus  20  includes a galvanometer scanner, and the linear polarizer  22  is a polarizing plate. A laser beam (labeled “Lb”) is output by the laser oscillator  21 , passes through the linear polarizer  22 , and is applied to the first thin film resistor  2  to thereby trim the first resistor  2  in a manner to be described. It will be appreciated that the laser apparatus  20  could output any suitable laser beam Lb, including but not limited to a linearly polarized laser beam Lb and a circularly polarized laser beam.  
         [0032]     Referring to  FIGS. 2B and 3 , the wafer  1  includes a first electrode  13  and a second electrode  14 . The first resistor  2  is interposed between the first electrode  13  and the second electrode  14 . More specifically, the first electrode is disposed adjacent the first side  3   a  of the first resistor  2 , and the second electrode  14  is disposed adjacent the second side  3   b  of the first resistor  2 . Also, a first pad  15  is electrically connected to the first electrode  13 , and a second pad  16  is electrically connected to the second electrode  14 . In one embodiment, the first pad  15 , the second pad  16 , the first electrode  13 , and the second electrode  14  are each made out of aluminum. Also, in one embodiment, the first pad  15  is integrally connected to the first electrode  13 , and the second pad  16  is integrally connected to the second electrode  14 .  
         [0033]     A first barrier metal layer  12   a  is interposed between the first electrode  13  and the first resistor  2 , and a second barrier metal layer  12   b  is interposed between the second electrode  14  and the first resistor  2 . As such, the first pad  15  is electrically coupled to the first edge  3   a  of the first resistor  2 , and the second pad  16  is electrically coupled to the second edge  3   b  of the first resistor  2 . Also, the first resistor  2  and the first and second electrodes  13 ,  14  are coated with a surface protection film  9  (e.g., SiO 2  film).  
         [0034]     Moreover, a measuring device  27  is included in  FIG. 4 . As shown in  FIG. 4 , a first probe  25  establishes electrical communication between the first pad  15  and the measuring device  27 , and a second probe  26  establishes electrical communication between the second pad  16  and the measuring device  27 . As such, the measuring device  27  is operable to measure the insulation resistance of the first resistor  2 .  
         [0035]     As shown in  FIG. 2A , the second thin film resistor  18  is intended to be laser trimmed to form a trim cut  19  (shown in broken lines in  FIG. 2A ). Before the trim cut  19  is formed, however, the proper parameters for laser trimming the second thin film resistor  18  are identified.  
         [0036]     To identify the proper parameters for laser trimming of the second resistor  18 , a first trim cut  17   a  (see  FIG. 4 ) is formed in the first thin film resistor  18 . In one embodiment, the first trim cut  17   a  is formed so as to extend in the same linear direction as the trim cut  19  intended for the second thin film resistor  18 . More specifically, in the embodiment shown in  FIG. 2A , the trim cut  19  is intended to extend in the Y-direction; therefore, the first trim cut  17   a  is likewise formed so as to extend in the Y-direction as shown in  FIG. 4 .  
         [0037]     Once formed, the first trim cut  17   a  bisects the first resistor  2  such that the first resistor  2  is divided into a first portion  4  and a second portion  5 . Specifically, the first trim cut  17   a  extends from the third edge  3   c  to the fourth edge  3   d , and the first trim cut  17   a  is approximately orthogonal to the third and fourth edges  3   c ,  3   d . The first trim cut  17   a  is interposed between the first and second pads  15 ,  16  so as to be approximately parallel to the first and second pads  15 ,  16 . As a result, the first pad  15  is electrically coupled to the first portion  4 , and the first pad  15  is separated from the second portion  5  by the first trim cut  17   a . In addition, the second pad  16  is electrically coupled to the second portion  5 , and the second pad  16  is separated from the first portion  4  by the first trim cut  17   a . It will be appreciated, however, that the first trim cut  17   a  could be formed in any suitable direction.  
         [0038]     Once the first trim cut  17   a  is formed, the measuring device  27  is used to detect electrical characteristics (e.g., the insulation resistance) of the first resistor  2 . This data can be used for evaluating the trim cut  17   a  and for evaluating the laser trimming process used to form the trim cut  17   a . For example, the data can be used to determine whether uncut parts are generated, whether the laser beam Lb is misfocused, whether the trimming window is adequate, and/or whether the laser intensity is adequate.  
         [0039]     For instance, an operative laser intensity for laser trimming the second resistor  18  is determined in one embodiment. To determine the operative laser intensity, the first resistor  2  is repeatedly laser trimmed at progressively increased laser intensities until the insulation resistance of the first resistor is approximately equal to a predetermined value. In other words, the first resistor  2  is repeatedly laser trimmed at progressively increased laser intensities until the insulation resistance of the first resistor  2  indicates that the trim cut is complete.  
         [0040]     More specifically, the first trim cut  17   a  is created with the laser beam Lb intensity set at a predetermined value. Then, the insulation resistance of the first thin film resistor  2  is measured. This measured insulation resistance is compared to the predetermined insulation resistance. If the measured insulation resistance is not approximately equal to the predetermined insulation resistance, then the laser intensity is increased, and an additional trim cut  17   b  is formed in the first resistor  2  as shown in  FIG. 5 . (As shown, the additional trim cut  17   b  is approximately parallel to the first trim cut  17   a  and bisects the first resistor  2  similar to the first trim cut  17   a .) Next, the insulation resistance of the first resistor  2  is measured again. If the insulation resistance is not approximately equal to the predetermined insulation resistance value, the laser intensity is further increased, and an additional trim cut  17   c  is formed as shown in  FIG. 5 . The laser intensity is increased and additional trim cuts are formed in the first resistor  2  until the insulation resistance is approximately equal to the predetermined insulation resistance value. At this point, the proper laser intensity is revealed, and the second resistor  18  is trimmed at that laser intensity.  
         [0041]     In one embodiment, for instance, trim cuts are formed in the first resistor  2  until the insulation resistance is approximately 100 MΩ or above. In other words, it can be expected that uncut parts remain in the trim cuts  17   a ,  17   b ,  17   c  if the insulation resistance is lower than 100 MΩ. Thus, as an example, a laser energy of 0.25 μJ is used to create the first trim cut  17   a . Then, the measuring device  27  detects that the insulation resistance of the first resistor  2  is 1 kΩ. This measured insulation resistance is lower than 100 MΩ, so an additional trim cut  17   b  is formed at a laser energy of 0.35 μJ. A measurement of the first resistor  2  reveals that the insulation resistance is above 100 MΩ, thereby revealing that the trim cut  17   b  is sufficiently complete (i.e., uncut parts are not present). Accordingly, the second resistor  18  is trimmed at the laser energy of 0.35 μJ or more such that the trim cut  19  ( FIG. 2A ) is complete and defects are unlikely to be created.  
         [0042]     It will be appreciated that if the insulation resistance is below 100 MΩ after forming the additional trim cut  17   b , an additional trim cut  17   c  is created at a higher laser intensity. This process of increasing the laser intensity, creating additional trim cuts, and measuring the insulation resistance is repeated until the insulation resistance of the first resistor  2  is approximately 100 MΩ or above. It will also be appreciated that if the first resistor  2  has an insulation resistance of 100 MΩ after the first trim cut  17   a  is formed, then laser trimming of the first resistor  2  is complete, and the trim cut  19  is formed in the second resistor  18  at the laser intensity used to form the first trim cut  17   a.    
         [0043]     The laser intensity can be adjusted by any suitable amount. When the laser intensity is adjusted by smaller increments, laser trimming and measurement will likely be performed a greater number of times before the appropriate laser intensity is discovered. However, the laser intensity discovered in the process may be more accurate. On the contrary, if the laser intensity is adjusted by larger increments, laser trimming and measurement will likely be performed fewer times before the appropriate laser intensity is discovered. However, the laser intensity discovered in the process may be less accurate.  
         [0044]     In one embodiment, trim cuts are evaluated (i.e., uncut parts are identified) and desirable laser intensities are identified by making trim cuts at a plurality of locations on the wafer  1 . For instance, in the embodiment shown in  FIG. 1 , trim cuts are made in nine different first resistors  2 .  
         [0045]     In one embodiment, each of the first resistors  2  is laser trimmed and a laser intensity adequate for trimming each of the first resistors  2  is identified. The highest laser intensity identified is then regarded as the laser intensity for trimming all of the second thin film resistors  18  on the wafer  1 .  
         [0046]     It will be appreciated that the laser intensity may vary depending on the position of the laser apparatus  20  relative to the surface of the wafer  1 . As such, a first resistor  2  and a second resistor  18  may be included in each IC chip on the wafer  1 , and the first resistors  2  are trimmed in the above-described manner to thereby identify a proper laser intensity for each IC chip. Then, the second resistors  18  are trimmed at the laser intensity identified for the particular IC chip. As a result, uncut parts are unlikely to be generated when laser trimming.  
         [0047]     Advantageously, the trim cuts  17  of the first resistor  2  can be evaluated more reliably. The trim cuts  17  are analyzed via an electrical analysis, rather than a visual analysis, which can be more accurate. Accordingly, the trim cut  19  of the second resistor  18  can be formed such that uncut parts are unlikely to be created in the trim cut  19 . Furthermore, the laser intensity for creating the trim cut  19  can be accurately determined before the trim cut  19  is created. As such, defects are less likely to be created.  
         [0048]     In one embodiment, the trim cut(s)  17   a ,  17   b ,  17   c  are formed in the first resistor  2  in a direction that corresponds to the direction of the trim cut  19  to be formed in the second resistor  18 . For example, in the embodiment described above and shown in  FIGS. 2A, 4 , and  5 , the trim cuts  17   a ,  17   b ,  17   c  formed in the first resistor  2  each extend in the Y-direction because the trim cut  19  of the second resistor  18  is to extend in the Y-direction as well. Alternatively, as shown in the embodiments of  FIGS. 7A, 7B ,  8 , and  9 , the trim cuts formed in the first resistor  2  extend in a direction that corresponds to trim cuts of the second resistor  18 .  
         [0049]     Specifically, in the embodiment shown in  FIG. 7A , the second resistor  18  includes a trim cut  19   b  that extends in the X-direction. Thus, as shown in  FIG. 8 , trim cuts  17   a ,  17   b ,  17   c  each bisect the first resistor  2  in the X-direction. More specifically, the first trim cut  17   a  bisects the first resistor  2  such that the first resistor  8  is divided into a first portion  4  and a second portion  5 , and the trim cut  17   a  is interposed between the first pad  15  and the second pad  16 . The first pad  15  is coupled to the first portion  4 , and the first trim cut  17   a  separates the first pad  15  from the second portion  5 . The second pad  16  is coupled to the second portion  5 , and the first trim cut  17   a  separates the second pad  16  from the first portion  4 . The additional trim cuts  17   b ,  17   c  are each approximately parallel to the first trim cut  17   a  and are disposed in spaced relationship relative to the first trim cut  17   a  along the Y-direction.  
         [0050]     Similarly, in the embodiment shown in  FIG. 7B , the second resistor  18  includes a trim cut  19   c  that extends in the α-direction. (The α-direction could be at any suitable angle relative to the X- and Y-axes.) Thus, as shown in  FIG. 9 , trim cuts  17   a ,  17   b ,  17   c  each bisect the first resistor  2  in the α-direction. More specifically, the first trim cut  17   a  bisects the first resistor  2  such that the first resistor  8  is divided into a first portion  4  and a second portion  5 , and the trim cut  17   a  is interposed between the first pad  15  and the second pad  16 . The first pad  15  is coupled to the first portion  4 , and the first trim cut  17   a  separates the first pad  15  from the second portion  5 . The second pad  16  is coupled to the second portion  5 , and the first trim cut  17   a  separates the second pad  16  from the first portion  4 . The additional trim cuts  17   b ,  17   c  are each approximately parallel to the first trim cut  17   a  and are disposed in spaced relationship relative to the first trim cut  17   a  in a direction normal to the α-direction.  
         [0051]     Furthermore, in the embodiment shown in  FIG. 7A , the second resistor  18  includes a first trim cut  19   a  extending in the Y-direction, and a second trim cut  19   b  extending in the X-direction. In order to identify an adequate laser intensity for each of the trim cuts  19   a ,  19   b  of  FIG. 7A , a first resistor  2  is laser trimmed in the Y-direction (e.g., the first resistor  2  shown in  FIG. 5 ), and another first resistor  2  is laser trimmed in the X-direction (e.g., the first resistor  2  shown in  FIG. 8 ).  
         [0052]     Likewise, in the embodiment shown in  FIG. 7B , the second resistor  18  includes a first trim cut  19   a  extending in the Y-direction, a second trim cut  19   b  extending in the X-direction, and a third trim cut  19   c  extending in the α-direction. In order to identify a proper laser intensity for each of the trim cuts  19   a ,  19   b ,  19   c  of  FIG. 7B , a first resistor  2  is laser trimmed in the Y-direction (e.g., the first resistor  2  shown in  FIG. 5 ) , another first resistor  2  is laser trimmed in the X-direction (e.g., the first resistor  2  shown in  FIG. 8 ), and still another first resistor  2  is laser trimmed in the α-direction (e.g., the first resistor  2  shown in  FIG. 9 ). As such, the second resistor  18  can be laser trimmed at a more accurate laser intensity, regardless of the direction for trimming the second resistor  18 .  
         [0053]     In one embodiment, the first pad  15  and the second pad  16  associated with the first resistor  2  are electrically isolated from other electrical components of the IC chip. In another embodiment, the first pad  15  and the second pad  16  are in electrical communication with other components of the IC chip. This embodiment is illustrated in  FIG. 6 .  
         [0054]     Specifically, in  FIG. 6 , the first pad  15  is a grounding pad that is in electrical communication with other components of a digital circuit. The second pad  16  is a grounding pad that is in electrical communication with other components of an analog circuit. It will be appreciated that the first and second pads  15 ,  16  may be other circuit elements, such as power supply pads, rather than grounding pads. As such, the first and second pads  15 ,  16  are in electrical communication with ground terminals (or power supply terminals) of separate circuits on the IC chip.  
         [0055]     The first resistor  2  is interposed between the first and second pads  15 ,  16  similar to the embodiment described above in relation to  FIG. 2B . Furthermore, the first pad  15  and the second pad  16  are at a predetermined electrical potential so as to function within the respective circuit(s) of the IC chip. Accordingly, the first and second pads  15 ,  16  can serve as electrodes for measurement of the insulation resistance of the resistor  2 , and the first and second pads  15 ,  16  can also serve as elements of a circuit within the IC chip, such as pads for the IC power supply or as grounding pads.  
         [0056]     Specifically, after the trim cuts  17   a ,  17   b ,  17   c  are created in the first resistor  2  and a suitable laser intensity is identified, the first and second pads  15 ,  16  are electrically connected to a lead frame  34  for grounding terminals (or a lead frame for power supply terminals) through bonding wires  32  and  33 . It will be appreciated that the first pad  15  and the second pad  16  are electrically insulated from each other as a result of trimming of the resistor  2 . Thus, a voltage fluctuation in the digital circuit is unlikely to affect the analog circuit, and a voltage fluctuation in the analog circuit is unlikely to affect the digital circuit.  
         [0057]     The present invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the present invention may be practiced other than as specifically described.