Abstract:
A system detects the clearing of a dielectric at a plurality of contact sites by measuring the surface voltage of the dielectric and comparing the surface voltage to a reference voltage set to a value that relates to the cleared contact sites. Another system detects the clearing of a dielectric at a plurality of contact sites on a substrate by measuring the rate of change of a substrate current during an etch process and ending the etch process when the rate of change is approximately zero. Another system detects the clearing of a dielectric at a contact site by measuring a substrate current during an etch process and ends the etch process when the measured substrate current exceeds a predetermined value.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to the field of semiconductor manufacturing, and more particularly, to the field of etching dielectrics. 
     BACKGROUND OF THE INVENTION 
     Types of dielectrics used in semiconductor manufacturing include oxides, nitrides, borophosphosilicate glasses (BPSG), silicon-dioxides, silicon-nitrides, and tetra-ethyl-ortho-silicates (TEOS). During an integrated circuit manufacturing process, these dielectrics are often etched. For example, insulating oxides are etched, protective oxides are etched, and sacrificial oxide masks are etched. Dielectrics sometimes function as insulators to isolate one level of conductors and devices from another. However, the conductors and devices on different levels must be interconnected in order to have a working integrated circuit. This is accomplished by etching holes in the dielectric layers in order to connect one layer to another. In the art of integrated circuit manufacturing, these etched holes are referred to as contacts or vias. In this document, all holes etched in a dielectric are referred to simply as contacts. 
     A long standing problem in the art of manufacturing integrated circuits is that of completing a process step and not knowing whether the process step completed successfully. If the step did not complete successfully, and the processing of the integrated circuit continues, then it is likely that at the end of the manufacturing process the circuit will not work as designed. Thus, continued processing after a failed process step results in wasting the costs of processing after the failed step. 
     In the etching of dielectrics, a problem that can cause a processing step to fail is the failure of the process to completely etch the dielectric at a contact location. This failure prevents devices from being connected. One approach to solving this problem is to design the etching process to over etch, i.e., to run the process longer than necessary for etching some contacts in order to completely etch all contacts on the substrate. One difficulty with this approach is that over etching results in some contacts being etched to dimensions larger than necessary, and this interferes with the important goal of integrated circuit manufacturing of increasing the density of the devices on a substrate. 
     For these and other reasons, there is a need for the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method for overcoming the problems as described above and others that will be readily apparent to one skilled in the art from the description of the present invention below. 
     A system in accordance with one embodiment of the present invention for use in identifying the successful completion of a dielectric etching process on a semiconductor substrate includes a voltage probe for measuring the surface voltage of the dielectric, a selectable reference voltage, and a comparator. The selectable reference voltage is set to a value related to the surface voltage of the dielectric when the contacts are cleared of the dielectric. The comparator is coupled to the selectable reference voltage and the voltage probe. The comparator compares the measured voltage to the selectable reference voltage and produces an endpoint detection signal. 
     In one embodiment of the system, the voltage probe is a non-contact probe. In another embodiment of the system, the selectable reference voltage is set to a value approximately equal to the surface voltage of the dielectric when the contacts are cleared of the dielectric. In still another embodiment, the comparator is an analog comparator, and in yet another embodiment, the comparator is a digital comparator. 
     A method in accordance with one embodiment of the present invention for identifying the completion of a dielectric etching process on a semiconductor substrate includes the steps of setting a selectable reference voltage to a value related to the surface voltage of the dielectric when a contact is cleared of the dielectric, measuring the surface voltage of the dielectric, comparing the measured voltage to the selectable reference voltage, and identifying the successful completion of the dielectric etching process by noting when the measured voltage is less than the selectable reference voltage. 
     In one embodiment of a method of the present invention, the selectable reference voltage is set to a value of approximately equal to the surface voltage of the dielectric when the contacts are cleared of the dielectric. In another embodiment, measuring the surface voltage of the dielectric consists of averaging multiple measurements of the surface voltage of the dielectric. 
     A method for etching a dielectric on a semiconductor substrate in a plasma etch system is also described. The method includes placing a substrate with a dielectric to etch within a plasma etch chamber, setting a selectable reference voltage to a value related to the surface voltage of the dielectric when the contact is cleared of the dielectric, etching the dielectric in the plasma etch chamber, measuring the surface voltage of the dielectric, generating an endpoint detection signal when the measured voltage is less than the selectable reference voltage, detecting the endpoint detection signal, and stopping the etching when the endpoint detection signal is detected. 
     In one embodiment of this method, the selectable reference voltage is set to a value approximately equal to the surface dielectric voltage when the contact is cleared of the dielectric. 
     In another embodiment, a method for etching a dielectric on a semiconductor substrate in a plasma etch system includes placing a substrate with a dielectric to etch within a plasma etch chamber, setting a selectable reference current to a value related to the substrate current when the contact is cleared of the dielectric, etching the dielectric in the plasma etch chamber, measuring the substrate current, generating an endpoint detection signal when the measured current is greater than the selectable reference current, detecting the endpoint detection signal, and stopping the etching process when the endpoint detection signal is detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a system in accordance with the present invention in which the contact dielectric etching is incomplete. 
     FIG. 2 is a block diagram of a system in accordance with the present invention in which the contact dielectric etching is complete. 
     FIG. 3A is a graph showing the relationship between the dielectric surface voltage and the selectable reference voltage for an area of a semiconductor substrate that has not been completely etched and an area of the semiconductor substrate that has been completely etched. 
     FIG. 3B is a graph showing the endpoint detection signal in an unetched area and an etched area. 
     FIG. 4 is a general flow diagram of the endpoint detection process of the present invention. 
     FIG. 5 is a general flow diagram of a second embodiment of the endpoint detection process of the present invention. 
     FIG. 6 is a general flow diagram of a method for real time detection of the endpoint of a dielectric etching process in a plasma environment of the present invention. 
     FIG. 7 is an illustration of a measurement system for measuring the surface voltage of a semiconductor substrate in a plasma etch chamber using a voltage probe. 
     FIG. 8A is an illustration of a system for sensing a substrate current in a substrate having partially etched contacts. 
     FIG. 8B is an illustration of a system for sensing a substrate current in a substrate having etched contacts. 
     FIG. 8C is an graph of a substrate current versus time for a plasma etch process of a substrate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     Embodiments of systems and methods in accordance with the present invention shall be described with reference to FIGS. 1-8. The embodiments of the systems and methods of the present invention for identifying the completion of a dielectric etching process on a semiconductor substrate are useful whenever contacts are etched in a dielectric, and determining whether the contacts are cleared of the dielectric is desired. Embodiments of the present invention can also be used in connection with processes that make use of protective oxides, sacrificial oxides, nitrides, borophosphosilicate glasses (BPSG), silicon-dioxides, silicon-nitrides, and tetra-ethyl-ortho-silicates (TEOS). 
     As shown in FIG. 1, in one embodiment of the present invention system  100  comprises voltage probe  110 , selectable reference voltage  120 , and comparator  130 . 
     Voltage probe  110  measures surface voltage  140  of dielectric  150  on semiconductor substrate  155  after a dielectric etching process. As one skilled in the art will recognize, any device that can sense surface voltage  140  of dielectric  150  is suitable for use in the present invention. In one embodiment, a non-contact Kelvin Probe is used to sense surface voltage  140 . A Kelvin Probe is a non-contact, non-destructive vibrating capacitor device used to measure the work function difference, or for non-metals, the surface potential, between a conducting specimen and a vibrating tip. Kelvin Probes are known to practitioners in the art of integrated circuit manufacturing. 
     A reference voltage, such as selectable reference voltage  120  is set to a value that corresponds to surface voltage  140  of dielectric  150  when contact site  160  is cleared of the dielectric during an etching process. The precise value for a given manufacturing step can be determined by measuring surface voltage  140  of dielectric  150  at the completion of a dielectric etching process and then verifying that contact site  160  is cleared of the dielectric using a scanning electron microscope. The precise value of the selectable reference voltage can depend on the physical parameters of the etching process, such as the initial depth of dielectric  150 , the number of contact sites  160  in dielectric  150 , and the aggressiveness of the etching process. In a typical process, with a dielectric thickness of one thousand angstroms, selectable reference voltage  120  can have a value of between one-half volt and two volts. 
     Comparator  130 , in one embodiment, is coupled to voltage probe  110  and selectable voltage reference  120  for the purpose of generating endpoint detection signal  170  shown as a time-voltage magnitude graph. Comparator  130 , in one embodiment, is an analog device with an analog output, and compares the voltage measured by voltage probe  110  with selectable reference voltage  120 . Endpoint detection signal  170  indicates whether the voltage measured by voltage probe  110  is greater than or less than selectable reference voltage  120 . In an alternate embodiment, comparator  130  is an analog integrated circuit comparator. In another embodiment, comparator  130  is a digital comparator. In still another embodiment, comparator  130  is a person who compares the surface voltage indicated by voltage probe  110  to selectable voltage reference  120 . The digital comparator can be implemented in a microprocessor, as a combination of hardware and software, or strictly in hardware. An analog comparator is preferable when voltage probe  110  and selectable reference voltage  120  generate analog voltage output signals, a digital comparator is preferable when selectable reference voltage  120  and voltage probe  110  generate digital output signals, and a human comparator is preferable when voltage probe  110  provides a visual displays of the voltages or a visual display of the relationship between the voltages. 
     FIG. 2 shows the system of FIG. 1 with like components labeled with like reference numerals. A difference between FIG.  1  and FIG. 2 is that in FIG. 1 contact site  160  is not cleared of the dielectric, while in FIG. 2 contact site  260  is cleared of the dielectric. Another difference is that surface voltage  240  of FIG. 2 has a value different from the value of surface voltage  140  of FIG.  1 . Still another difference is that FIG. 2 shows endpoint detection signal  270  as a time-voltage magnitude graph assuming a positive voltage level, which indicates that contact  260  is cleared of the dielectric. Whereas, FIG. 1 shows endpoint detection signal  170  assuming a low voltage level, indicating that contact  160  is not cleared of the dielectric. 
     FIG. 3A shows in graphical form the relationship between surface voltage  140  of FIG.  1  and selectable reference voltage  120  in an area of a semiconductor substrate that has not been completely etched, unetched area  180 , and the relationship between surface voltage  240  of FIG.  2  and selectable reference voltage  120  in an area of a semiconductor substrate that has been completely etched, etched area  190 . In the unetched area  180 , which is related to FIG. 1, the etching process has not cleared dielectric  150  from contact site  160 . As shown in FIG. 3A, in the unetched area  180 , surface voltage  140  is greater than selectable reference voltage  120 , and as shown in FIG. 3B, endpoint detection signal  170  is at a low level. In etched area  190 , which is related to FIG. 2, the etching process has cleared dielectric  250  from contact site  260 . Also, as shown in FIG. 3A, in etched area  190 , surface voltage  240  is less than selectable reference voltage  120 , and as shown in FIG. 3B endpoint detection signal  270  is at a high level. Endpoint detection signal  170  may be implemented in positive logic as in FIG. 3B or in negative logic, in which case the polarity of endpoint detection signal  170  is complemented. 
     In operation, surface voltage  140  and surface voltage  240  stabilize after the etching process completes. In some manufacturing process environments, stabilization occurs a few minutes after completion of the etching process, while in other environments stabilization may not occur for an hour or more after completion of the etching process. The actual stabilization time is determined empirically for each process etch step in the manufacturing of a particular product and may depend on environmental factors. After stabilization, system  100  measures surface voltage  140  as shown in FIG. 1 or surface voltage  240  as shown in FIG.  2 . After the measurement is taken, system  100  compares the measured value to selectable reference voltage  120 . Selectable reference voltage  120 , of FIG. 2, is set to a value that can be obtained empirically and is related to the surface voltage  240  of the dielectric  250  when the contact site  260  is cleared of the dielectric. If the contact site  160 , of FIG. 1, is not cleared of the dielectric, then, as illustrated in FIG. 1, the endpoint detection signal is maintained at a low level. If the contact site  260 , of FIG. 2, is cleared of the dielectric, then, as illustrated in FIG. 2, the endpoint detection signal  270  assumes a high level, indicating that the dielectric etching process completed successfully. An advantage of system  100  is that at the completion of the dielectric etching process, system  100  makes determining the success or failure of the process relatively easy. 
     An embodiment of a method in accordance with the present invention is shown in FIG.  4 . Method  400  for identifying the completion of a dielectric etching process includes setting  410 , measuring  420 , comparing  430 , and identifying  440  operations. In the setting  410  operation, a selectable reference voltage is set to a surface voltage value, which indicates that the dielectric at the contacts is cleared. In the measuring  420  operation, a voltage probe measures the surface voltage of the dielectric after the dielectric etching process in order to obtain the value of the surface voltage prior to the comparing  430  operation. The measuring  420  operation is preferably performed after the surface voltage has stabilized following the etching process. The surface voltage, after the etching process, is an indicator of whether the etching process completely etched the dielectric at the contact site. In the comparing  430  operation, the measured surface voltage is compared to the selectable reference voltage. And in the identifying  440  operation, when the measured voltage is less than the selectable reference voltage, an indicator of whether the dielectric etching process completed successfully is generated. 
     An advantage of this embodiment is that it can be tailored to dielectric etching steps at any point in the manufacturing process. This is accomplished by determining the reference voltage for a given process through measuring the surface voltage after the completion of the process and stabilization of the surface voltage, and by verifying that the contact site is cleared. One method of verifying that the contact site is cleared is to observe the contact site using a scanning electron microscope. 
     An alternate embodiment of the present invention is shown in FIG.  5 . The method includes setting  510 , measuring  520 , averaging  545  comparing  530 , and identifying completion  540  operations. As will be recognized by those skilled in the art, it is possible for a single measurement to be in error. So, for the purpose of increasing the accuracy and reliability of the measurement of the surface voltage, the embodiment shown in FIG. 5 adds the averaging  545  operation for averaging multiple surface voltage measurements. The number of measurements to average may be determined empirically using methods known in the art. 
     In another embodiment of the present invention, a further improvement in the surface voltage dielectric measurement process is achieved when the measurements are made at multiple locations on the dielectric. As will be appreciated by those skilled in the art, local process variations in the semiconductor manufacturing process are common and can be accounted for by making multiple measurements at different locations on the surface of the substrate. 
     FIG. 6 shows a general flow diagram of method  600 , a real time embodiment of the present invention. An advantage of the embodiment of method  600  is that time is not wasted making measurements after completion of the dielectric etching process. Method  600  comprises placing  610 , setting  620 , etching  630 , measuring  640 , generating  650 , detecting  660 , and stopping  670  operations. 
     Referring to FIG. 6, the placing  610  operation requires placing a substrate having a dielectric to etch within a plasma etch chamber. The setting  620  operation requires setting a selectable reference voltage as described in the previous embodiments of the invention. In one embodiment of the present invention, the selectable reference voltage is set to a value approximately equal to the surface voltage of the dielectric when a contact site is cleared of the dielectric. The etching  630  operation requires etching the dielectric in the plasma etch chamber. The measuring  640  operation requires measuring the surface voltage of the dielectric. Any method known to those skilled in the art for measuring a surface voltage in real time is suitable for use in connection with the present invention. The generating  650  operation requires generating an endpoint detection signal when the measured surface voltage is less than the selectable reference voltage. The endpoint detection signal is generated by a comparator as described in the previously described embodiments of the invention. The detecting  660  operation requires detecting the endpoint detection signal. In a positive logic system, the endpoint detection signal is detected by identifying the time when the endpoint detection signal goes positive. The stopping  670  operation requires stopping the etching process when the endpoint detection signal is detected. Purging the plasma chamber or removing the substrate from the plasma etch chamber stops the etching process. 
     FIG. 7 shows a measurement system  700  for measuring the surface voltage or a substrate current of substrate  715  in plasma etch chamber  705  using probe  710 . The present invention can be practiced in connection with a variety of embodiments of probe  710 . For example, in one embodiment, probe  710  is a voltage probe, in another embodiment probe  710  is a circuit capable of sensing current or the rate of change of a current signal, in still another embodiment probe  710  is an ammeter or a calibrated ammeter, and in yet another embodiment probe  710  is a computer system capable of measuring current or the rate of change of a current signal. 
     Various embodiments of processes and systems for measuring the surface voltage have been described above. Some of these processes and methods can be used in connection with the measurement of a surface voltage of semiconductor substrate  715 . Measurement system  700  has the advantage that semiconductor substrate  715  is not removed from plasma etch chamber  705  before making a surface voltage measurement, and therefore reduces the overall manufacturing time for the substrate. 
     Referring to FIG. 8A, in current sensing system  800 , plasma ions  803  are capable of inducing a current  806  in substrate  809 . Substrate  809  is not limited to a particular material. In one embodiment, substrate  809  is a semiconductor, such as silicon. In an alternate embodiment, substrate  809  is gallium arsenide. As long as the plurality of contacts, such as contact  812  and contact  815 , are not cleared of material  818 , current  806  is likely to be relatively small, in the range of picoamperes. Current  806  is sensed by current sense device  821 , which can assume a variety of embodiments. For example, current sense device  821  can be a circuit, an ammeter, a calibrated ammeter, or a computer system capable of sensing current. Material  818  is generally a dielectric. Types of dielectrics suitable for use in connection with the present invention include oxides, nitrides, borophosphosilicate glasses (BPSG), silicon-dioxides, silicon-nitrides, and tetra-ethyl-ortho-silicates (TEOS). Referring to FIG. 8B, in current sensing system  823 , as contacts  827  and  830  are cleared, substrate current  833 , which is induced by plasma ions  836 , increases to a relatively large value in the range of microamperes or milliamperes. This current can be measured using current sense device  839 . In one embodiment, current sense device  839  is a circuit. In another embodiment, current sense device  839  is an ammeter. In yet another embodiment, current sense device  839  is a computer system capable of sensing current. 
     Referring to FIG. 8C, a substrate current versus time graph  841  shows the increase in current along line  844  as time changes from the beginning of an etch process at time zero  847  until etch finish time  850 . At etch finish time  850 , the rate of change of the current approaches zero. In one embodiment of the present invention, this rate of change is detected to identify etch finish time  850 . In an alternate embodiment, etch finish time  850  is detected by empirically determining the current value at which the etch process is complete. Substrate current at etch process time zero  847  is on the order of picoamperes and at etch finish time  850  substrate current is on the order of microamperes or milliamperes. As described above, the substrate current value at etch finish time  850  is determined by etching a substrate, measuring the substrate current, and verifying that the contacts are cleared using a scanning electron microscope. 
     It is to be recognized that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     CONCLUSION 
     The identification by the applicant of the relationship between the dielectric etching process and the surface voltage, and the real time relationship between the dielectric etching process and the substrate current, permits the above described embodiments of the present invention. The embodiments exploit the process insight that as a contact site is cleared of dielectric, the surface voltage of the dielectric decreases, and that in real time as a contact site is cleared of dielectric, the substrate current increases. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.