Patent Publication Number: US-6987067-B2

Title: Semiconductor copper line cutting method

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to a method for cutting copper lines embedded in a semiconductor chip. 
   BACKGROUND OF THE INVENTION 
   It is well known in the semiconductor manufacturing art that the extremely tiny semiconductor components (hereinafter referred to as “chips”) are generally made up of a base substrate material, such as silicon. Disposed on top of that base are layers of insulation, such as silicon dioxide, and conductive materials, such as metals, which form the complex circuitry of the chip. If one were to examine a delaminated or cross sectioned modem semiconductor chip, which would necessarily require many degrees of magnification, one would see a complex pattern of circuit devices, such as transistors, and multi-leveled, crisscrossing wiring that connects the circuit devices together. For may years, one such metal which was typically used in connection with the manufacture of semiconductors was aluminum. In the last several years, however, copper has become a viable substitute for aluminum in this application. 
   As is known to those skilled in the art, the actual manufacture of a semiconductor chip is a complex and time consuming process, involving many steps which are dependent upon the usage of tools, materials, and semiconductor masks. Accordingly, once a given chip is manufactured, if the chip proves to be defective, due, for example, to a given wire on the chip making an incorrect contact, it is not easy or expedient to simply create a revised set of masks, and manufacture a new batch of chips. Instead, significant delays may be incurred between the time the wiring problem is detected and actual manufacture of a new chip with correct wiring. Any such delay can have a significant negative effect on the launching of a commercial computer product that is dependent upon the chip because other designers involved with the launch of the complete product may be delayed in doing their work on the project. For example, there may be a staff of computer software programmers waiting to have access to the chip so that they can create software that will run on the chip. Hence, delays in being able to access an actual chip may impact their schedule, which can then have a ripple effect on other stages of the project, too. 
   To address this overall problem, it is known in the semiconductor manufacturing art to repair a defective chip after it is manufactured. For example, see U.S. Pat. Nos. 5,171,709 and 5,182,230 both to Donelon et al., generally describing the processes for repairing integrated circuits. Through the repair process individual wires inside the chip may be exposed, cut, rerouted, and sometimes reconnected to other wires. Due to the extremely tiny size of these individual wires, the repair process requires considerable precision and time. Thus, the repairing of individual chips is generally not a very cost effective approach to fixing problems with a commercial quantity of chips, but the repair process is suitable for fixing problems with a much smaller number of defective chips, like the number that may be manufactured as prototypes. Importantly, the repair of a relatively small number of prototype chips is a process that addresses the issue discussed above, namely it enables the defective chip to be fixed well enough so that it can be made available for further viability testing and used by other members of a system design team, such as the software programmers. 
   The repair process generally starts with the determination of what part of the chip needs to be reworked. For example, it may be determined that a given wiring connection should not be made, in which case the objective of the repair would be to cut and terminate the given conductive line on the chip. Or, it may be determined that a given wiring connection should be re-routed to make contact with a point to which it was not originally connected, in which case the objective of the repair would be to cut the given conductive line and reconnect it to the different point. Hence, in many rework processes, the insulative materials of the chip must be selectively removed to expose the conductive wires that need to be cut. Then, the wires themselves need to be selectively cut, too. In order to do the selective removal of the insulative material and the conductive wire, the chip is generally placed in a chamber, into which various gases may be pumped. With the introduction of the right gases, a focused ion beam (FIB) is then used to precisely cut and remove the materials that need to be removed in order to accomplish the repair. Thus, it is the combination of the chemical properties of the gases with the materials of the chip, as made reactive by the ion beam, that all serve to accomplish the selective cutting and removal of materials. 
   As the semiconductor industry transitioned from aluminum conductive wiring to copper conductive wiring on chips, certain problems developed when attempting to repair chips containing copper. Specifically, gases that had been used successfully in connection with the removal of aluminum conductive lines were proven to be unsuccessful in connection with the removal of copper conductive lines. For example, certain gases customarily used with aluminum were found to spontaneously react with copper, by which is meant that they were not made to react only when excited by an ion beam, but instead they reacted with the copper even without ion beam excitation. Consequently, the use of such gases make it impossible to precisely control the copper removal process, which is essential. Still other gases typically used with aluminum have a tendency to cause copper to evaporate, but then deposit on the sidewalls of the hole that exposed the copper line. This is not desirable since it may create electrical contacts and shorts where they are not intended. Instead it is preferred that any copper that is cut by the ion beam form a reactive specie with the gas in the chamber so that it can be pumped out of the chamber. Yet another problem with known approaches is that certain gases which might work well with copper may have a tendency to attack and damage the internal components of the chamber, which also proved unworkable. 
   In view of the foregoing, what is needed is a method for cutting copper wires in a semiconductor chip, and in particular a method which is precise and which does not damage the chamber in which the cutting process takes place. 
   SUMMARY OF THE INVENTION 
   In accordance with the preferred embodiment of the present invention, a method for cutting a copper line (wire) on a semiconductor chip is provided. More specifically, the preferred embodiment of the method involves the use of a FIB system, which generally includes a chamber into which gas can be pumped while an ion beam (such as gallium) is directed at the specific target, namely the chip. At the commencement of the process the chip is placed in the FIB chamber. Then, gases (such as XeF 2 ) are introduced into the chamber and the ion beam is targeted at the chip for the purpose of removing the dielectric layers of the chip, which are immediately above the portion of the copper line that is to be cut. In this manner a hole is essentially drilled in the chip down to the copper line. Once the copper line is exposed, a combination of nitrogen (N 2 ) and oxygen (O 2 ) are bled into the chamber. With the N 2 /O 2  gas in the chamber the ion beam is again targeted at the chip this time for the purpose of cutting the copper line. 
   It is therefore an object of the present invention to provide a method for cutting copper lines on semiconductor chips. 
   It is also an object of the invention to provide such a method which is controlled and precise, and does not cause a spontaneous reaction between the gas used and the composition, including the copper, of the semiconductor. 
   It is another object of the invention to provide such a method which does not damage the chamber in which the cutting process takes place. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a FIB system in which the preferred embodiment of the invention is practiced on a semiconductor chip containing copper wiring. 
       FIGS. 2A-C  are a sequence of illustrations showing further details of the semiconductor chip in  FIG. 1  as it is operated on by the process of the preferred embodiment. 
       FIG. 3  is a flow diagram outlining the major steps of the process of the preferred embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
   The preferred embodiment is directed to a method for cutting copper lines embedded in a semiconductor chip; however, the disclosed process is generally adapted for the removal of any type of copper structure on a semiconductor chip. The environment in which the preferred embodiment is practiced is generally shown in  FIG. 1 , which presents an overview of a Focused Ion Beam (FIB) system  10 . Such FIB systems  10  are commercially available and are commonly used throughout the microelectronics industry as “chip repair” tools. The models 9100 and 9500 manufactured by Micrion, a division of FEI Co., would be suitable FIB systems for use with the preferred embodiments discussed in connection with this detailed description. As shown in  FIG. 1 , FIB system  10  includes an evacuated envelope  12  having an upper portion  14  within which are located a liquid metal ion source  16  and a focusing column  18  which includes extractor electrode means and an electrostatic optical system. Ion beam  20  passes from the liquid metal source  16  through column  18  and between electrostatic deflection plates  22 . Electrostatic deflection plates  22  direct the beam at chip  24 , which is a semiconductor device positioned on a stage  26  within chamber  28 . In the preferred embodiment, chip  24  is a semiconductor device manufactured to include layers of conductive copper lines sandwiched in between intermediate layers of insulation, such as silicon dioxide. 
   FIB system  10  further includes an ion pump  30  that is employed for evacuating the upper portion  14 . The chamber  28  is evacuated, preferably with a turbo molecular and mechanical pumping system  32  under the control of vacuum controller  34 . High voltage power supply  36  is connected to the liquid metal ion source  16  as well as to appropriate electrodes in the ion beam focusing column  18  for forming an ion beam  20  and directing the same downwardly toward chip  24 . Deflection controller and amplifier  38 , operated in accordance with a prescribed pattern, provided by pattern generator  40 , is coupled to deflection plates  22  whereby ion beam  20  may be controlled to trace a defined pattern on chip  24 , wherein that traced pattern will determine which part of chip  24  is going to be cut. 
   The ion source  16  typically provides a metal ion beam of gallium (although other sources are possible and have been discussed in the literature as an alternative to gallium). The high voltage power supply  36  provides an appropriate acceleration voltage to electrodes in ion beam focusing column  18  for energizing and focusing ion beam  20 . An electron multiplier  42  used for detecting secondary emission for imaging is connected to a video circuit and amplifier  44 , the latter supplying the drive for video monitor  46  also receiving deflection signals from deflection controller and amplifier  38 . As will be detailed below, in order to remove the copper from chip  24  two different chemicals (N 2  and O 2 ) need to be present on the surface of chip  24  at the same time. In the preferred embodiment, a commercially available FIB tool was equipped with several separate gas chemistry delivery channels. This allows for the mixing of these gases to actually take place at the point of delivery within the chamber. Further, it enables variable gas concentrations and ratios for the purpose of process optimization. 
   Each channel originates with a chemical source. The source can start out in any phase-solid, liquid, or gas. In all cases the chemistry needs to be delivered to the chamber in the gaseous phase, but a gas can be derived from a solid or liquid thru sublimation or evaporation, respectively. Within each channel the derived gas is routed through a mass flow controller with variable set point and feedback for providing a suitable final vapor pressure in accordance with art recognized practices. Each channel terminates within the confines of the chamber with a separate gas delivery nozzle, which is generally designated as nozzle cluster  62  in FIG.  1 . The multiple individual nozzles in nozzle cluster  62  allow any or all of the gases available to be released onto the surface of chip  24  at the same time. A single extension of the cluster assembly places all of the gas nozzles in close proximity to the chip surface. 
   In the preferred embodiment, nozzle cluster  62  comprises four capillary tubes which extend from the roof of the chamber  28 , and are connected thru control valves and flow controllers to the individual chemical source reservoirs, together generally designated as gas box and controls  53 . These serve the purpose of delivering precise amounts of gaseous vapor. The nozzle assembly is extended and translated in orthogonal directions substantially perpendicular to its axis employing translation apparatus  54 , so that gaseous vapor can be aimed directly toward a region on the top surface of chip  24 . 
   Access to the inside of chamber  28  is provided by a loadlock sample exchange port  66 . Port  66  is separated from the main chamber  28  by a gate valve  67 , which is large enough to allow the sample (chip  24 ), the sample holder, and the transport mechanism to pass into the interior of chamber  28 . Door  68 , when opened, allows the insertion of chip  24  into the pump down chamber (loadlock exchange port). A roughing pump draws a vacuum in this outer chamber until it begins to approach the pressure of chamber  28 . A valve between the chambers equilibrates the vacuum level, then the gate valve opens allowing chip  24  to pass from the outer to the inner chamber. A column isolation valve may be present to seal off and protect the ion source and focusing column apparatus in the event of chamber vacuum loss. 
   Having described the general components of FIB system  10 , further specifics are now provided on the processing of chip  24 . As indicated above, the preferred embodiment of the process is applied in an environment in which chip  24  is made up of a semiconductor device having one or more copper lines that need to be cut for purposes of chip repair. Accordingly,  FIGS. 2A-C  provide a sequence of illustrations showing the effects of processing chip  24 , in accordance with the preferred embodiment, inside chamber  28  of the FIB system  10 , shown in FIG.  1 . As will be noted from a quick look at  FIGS. 2A-C , the illustration of chamber  28  of FIB system  10  has been greatly simplified relative to  FIG. 1  (including leaving some specific components out of the pictures) to better illustrate the details of the preferred embodiment. Likewise, the scale and dimension of chip  24  has been greatly exaggerated to facilitate the illustration of germane aspects of the preferred method. Finally, in addition to the illustrations in  FIGS. 2A-C ,  FIG. 3  provides a flow diagram showing the major steps involved in practicing the method of the preferred embodiment. 
   Turning now to  FIG. 2A , the simplified view of chamber  28  is provided, showing stage  26 , nozzle cluster  62 , and ion source  16 . At the commencement of the process, chip  24  would be placed in chamber  28  on stage  26  (step  300 , FIG.  3 ). In  FIG. 2A  (as well as the other figures in the sequence) chip  24  is shown in a side, sectioned view. Through the sectioning, chip  24  (which is highly simplified for purposes of illustration) is shown to be made up of a silicon substrate  100 , on top of which is positioned a layer of an insulative dielectric, which in the preferred embodiment is a layer of silicon dioxide  102 . Disposed vertically on one side of chip  24  is contact stud  104   a , and on the other side of chip  24  is contact stud  104   b . Studs  104   a  and  104   b  may be made of copper, or they can be made of other metals such as tungsten. Connecting studs  104   a  and  104   b  is a horizontal conductive line  110 , which is essentially a conductive wire and is made of copper. Disposed around studs  104   a ,  104   b , and line  110  is yet another layer  112  of insulative silicon dioxide. Finally, the top layer of passivation for chip  24  is layer  114  comprised of silicon nitride. Newer chip designs may substitute any one of a number of low-k dielectric material for the silicon dioxide layers surrounding the copper lines. 
   Having described the layers and elements of (simplified) chip  24 , it will now be assumed, for purposes of the further detailed discussion of the preferred embodiment, that it has been determined that chip  24  needs to be repaired. Specifically, it has been determined that when chip  24  was originally manufactured, the connection between stud  104   a  and  104   b  was made in error, and therefore in order to repair chip  24  copper line  110  connecting stud  104   a  and  104   b  needs to be cut. Accordingly, the balance of the discussion in this detailed description is on how that is accomplished using the resources of FIB system  10 . 
   In order to gain access to line  110  the layers above line  110  need to be removed to expose it to processing. Accordingly, layer  114  of silicon nitride and layer  112  of silicon dioxide must first be removed by means of processing in FIB system  10 . With respect to the processing parameters of FIB system  10 , in the preferred embodiment, a Micrion model 9100 was used, wherein it was equipped with a 30 keV gallium ion source, thereby producing a 10 nanometer minimum beam spot size. The beam current was varied from 50 to 250 picoamps It should be noted that although the preferred embodiment is based upon a 10 nm beam, in an alternate embodiment, a Micrion model 9500 was used as FIB system  10 , wherein it was equipped with a 50 keV gallium ion source, thereby producing a 5 nanometer minimum beam spot size. When shifting over to the 5 nm beam spot some minor beam scan parameters needed to be adjusted to account for the different spot size and beam current density. Such adjustments related to the beam dwell time, pixel spacing, end-of-raster gas refresh time, and the like, all such adjustments would be known to one skilled in the art. In FIB system  10 , chamber  28  had an initial base pressure of 10 −7  torr which was eventually dropped to 10 −5  torr when a flow of the reactive gas in the chamber commenced. It should be noted that in FIB system  10  pressure is monitored a considerable distance from chip  24 , so it may not accurately reflect the pressure in the immediate reactive environment. It is believed, however, that the actual gas concentration in the immediate area of chip  24  is on the order of 10 −3  to 10 −4  torr, or so. Processing is done at ambient room temperature with no RF or magnetic fields applied. All reaction kinetics are supplied by surface collision energy transfer from the primary beam  20  from ion source  16  striking the surface of chip  24 . 
     FIG. 2B  provides the next illustration in the sequence showing the removal of layer  114  and  112 , which is the process by which a hole  118  (the so-called “elevator shaft”) is cut to expose line  110 . Using the process parameters discussed above, in order to remove nitride and oxide layers  114  and  112 , a xenon difluoride (XeF 2 ) gas  120   a  is introduced into chamber  28  by nozzle cluster  62  (step  302 , FIG.  3 .). A gallium ion beam  20  is then directed at the top surface of chip  24 . As discussed above in connection with  FIG. 1 , the nozzles (not individually shown in  FIG. 2A-C ) can be directed at the location at which beam  20  contacts chip  24 , thereby concentrating gas  120   a  in that local to provide the chemistry for ion beam etching. It should be noted that SiLK and other organic interlevel dielectrics may require some other techniques, known to those skilled in the art, to achieve the best drilling profiles. These usually include varying the primary beam (gallium) parameters to make the XeF 2  etch efficient, or include changing the chemistry, such as by introducing in chamber  28  a water vapor. As shown in  FIG. 2B , and using the process parameters discussed above, a 1.0 um diameter “elevator shaft” (hole  118 ) was drilled thru 4.5 um of the dielectric comprising layers  114  and  112 , using the XeF 2  gas (step  304 , FIG.  3 ). It should also be noted that during the removal of layers  114  and  112 , the vacuum of pumping system  32  (shown in  FIG. 1 ) will be operational for the purpose of drawing the gases and vaporized dielectric composition of these layers out of chamber  28 . Hence, as a result of this vacuum operation, chamber  28  should be cleared of any residual gases and vapors from the drilling of hole  118 , and the base pressure of the chamber should be reestablished before the next process step commences (step  306 , FIG.  3 ). 
   Having exposed copper line  110  through the procedure described above, the next objective of the process is to remove a portion of copper line  110 , for the purpose of severing the connection between stud  104   a  and stud  104   b .  FIG. 2C  provides the next illustration in the sequence. In connection with the cutting of copper line  110  the FIB system 10 parameters discussed above are generally maintained; however, at this stage of the process nozzle cluster  62  will introduce a gas  120   b , which is a combination of nitrogen (N 2 ) and oxygen (O 2 ), into chamber  28  (step  310 , FIG.  3 ). Again the nozzles of nozzle cluster  62  would be directed at the location where the gallium ion beam  20  contacts chip  24 , thereby enhancing the beam etching process. As seen in  FIG. 2C , beam  20  reacting together with the (N 2 /O 2 ) gas will etch away copper line  110 , thereby accomplishing the objective of cutting the electrical connection between stud  104   a  and  104   b  (step  312 , FIG.  3 ). 
   In the preferred embodiment, gas  120   b  is an equal proportion of nitrogen and oxygen in the form of N 2 /O 2 . The use of the combination of nitrogen and oxygen as the gas  120   b  in chamber  28  for the cutting of a copper line  110 , has been found to yield a number of significant advantages. For example, N 2 /O 2  has consistently yielded clean, uniform cuts in the copper material, with little or no sign of redeposition of the copper or copper by-products on the sidewalls of hole  118  in the dielectric layers  112  and  114 . This is desirable for the purpose of insuring that no unintended electrical contact structures form in chip  24  as a result of the copper removal process. Also, electrical tests confirm that the process yields a successful cut through the copper such that, in the example shown, the line  110  between stud  104   a  and  104   b  is truly eliminated. Furthermore, in connection with the development of the preferred embodiment, testing was done on exposed copper films. Those tests revealed that when a gaseous combination of nitrogen (N 2 ) and oxygen (O 2 ) are introduced to copper, and then targeted with the ion beam, there was clean etching in the specific target area with no spontaneous attack outside of the target region. This suggests that active species were only formed in the presence the ion beam and that the defined process is, indeed, a beam driven reaction. This being the case, it is also logical to conclude that no damage will occur elsewhere in the process chamber. For these many reasons, the combination of nitrogen (N 2 ) and oxygen (O 2 ) for use in the ion beam chamber when cutting copper has been found to be highly advantageous. Furthermore, in the preferred embodiment, gas  120   b , being an equal proportion of oxygen and nitrogen, was obtained through the use of a two channel gas system in which mixing occurred at the point of delivery. As these two gasses are chemically compatible, it would be possible to obtain a pre-mixed source vessel and deliver them to chip  24  through a single gas in the form of N 2 /O 2 . 
   The process of the preferred embodiment ends with the cutting of copper line  110 . It will be clear to those skilled in the art that after line  110  is so cut, it can be grounded or it can be rerouted to make connection with other electrical elements on chip  24 . Accordingly, once copper line  110  is cut as described additional process steps can be undertaken to simply fill the existing hole in chip  24  with a dielectric material or to add in other conductive material to establish new and different electrical connections. Such additional processing steps are not encompassed within the scope of this detailed description. 
   Also, it will be appreciated by those skilled in the art that the method of the present invention is not limited to the preferred embodiments discussed above. In particular it will be known by those skilled in the art that modifications can be made without departing from the spirit of the invention. For example, the exact mixture of nitrogen (N 2 ) and oxygen (O 2 ) can be varied so that they are not necessarily equal. Also, different types of FIB systems can be used with corresponding adjustments to their specific process settings. Furthermore, the overall process can be adapted in the event that the insulative dielectric covering the copper is not silicon dioxide, but is instead some alternate material, such as a low K dielectric. In any such event, the process can be adjusted to drill the elevator shaft through such alternate material, and then use the nitrogen (N 2 ) and oxygen (O 2 ) gas together with the ion beam to cut the copper. Further still, the process of the preferred embodiment can be used to remove not simply a copper line, but any copper structure, such as a copper contact or stud. Accordingly, any such modifications are considered within the scope of the invention, and the invention is limited solely by the appended claims.