Abstract:
Methods for detecting the endpoint of a photoresist stripping process provide O for reaction with the photoresist for a wafer to be stripped of photoresist. NO is also supplied for reaction with O not reacted with the photoresist. After substantially all the photoresist is stripped from the wafer, the rate of a reaction of O and NO to form NO 2  increases, which increases the intensity of emitted light. An operation of detecting this increase in light intensity signals the endpoint of the photoresist stripping process.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This Application is a divisional of prior application Ser. No. 09/468,742, filed Dec. 12, 1999 now U.S. Pat. No. 6,451,158 from which priority under 35 U.S.C. Section 120 is claimed. The entire disclosure of the prior application from which a copy of the declaration is herewith supplied is considered as being part of the disclosure of this Application and is hereby incorporated by reference therein. 
    
    
     BACKGROUND OF THE INVENTION 
     This disclosure relates generally to photoresist stripping processes, and more particularly to a method and apparatus for detecting the endpoint of a photoresist stripping process. 
     Fabrication of integrated circuits generally starts with a thin slice of silicon called a wafer. On this wafer one can fabricate several hundred individual chips. Each chip may contain 10 to 20,000 components for a total of several million devices per wafer. The devices include resistors, diodes, capacitors and transistors of various types. The fabrication of the devices includes depositing desired materials (such as silicon dioxide, aluminum, etc.) at certain locations. 
     A technique called photolithography is used to facilitate the introduction of materials at desired locations on the wafer and the removal of undesired material at other locations. As an example, a layer of aluminum is first deposited on the wafer. Next, the wafer is coated with a light sensitive polymer called photoresist. A mask is used to expose selected areas of photoresist to UV light. The UV light induces polymerization in the exposed photoresist. UV light causes the exposed photoresist to cross link, rendering it insoluble in developing solution. Such a photoresist is called a positive photoresist. A negative photoresist shows an opposite behavior. That is, exposure to UV makes the photoresist soluble in developing solution. After the exposure to light, the soluble portions of the photoresist are removed, leaving a pattern of photoresist. 
     Immediately after photolithography, the wafer with patterned photoresist is aluminum etched to remove the aluminum where there is no pattern. This has the effect of transferring the pattern to the aluminum, creating electrical connections among devices at different locations. 
     After the aluminum etch process is complete, the photoresist is removed from the wafer in a process called photoresist stripping. The stripping of photoresist from the wafer surface must be essentially complete, since photoresist left on the wafer surface will cause defects in the integrated circuit. An important consideration in the photoresist stripping process is determining a time, referred to as the endpoint, at which to end the process. This time must be chosen so that the photoresist is entirely removed from the wafer. Preferably, the time will not exceed the time when the photoresist has been entirely removed, since this decreases the efficiency of the fabrication of integrated circuits. 
       FIG. 1  is a flow chart of a prior art method for stripping photoresist  2 . The prior art method begins in a step  4 . The photoresist stripping process includes introducing a flow of O atoms into a stripping chamber that holds the wafer. The O atoms react with the photoresist, removing the photoresist from the wafer. The products of this reaction are removed from the chamber in the flow of gases leaving the chamber. The prior art method  2  continues the photoresist stripping process for a predetermined time in a step  6 , and ends the photoresist stripping process in a step  8  after the predetermined time elapses. The predetermined time is chosen to ensure that enough time passes to ensure that essentially all the photoresist has been stripped from the wafer. 
     One problem with the prior art method shown in  FIG. 1  is that it provides no means of detecting if all of the photoresist has been stripped from the wafer. Even if the photoresist has not been completely stripped from the wafer, the prior art method still stops the stripping process after the predetermined time has elapsed. In such a case, the presence of photoresist on the wafer will not be discovered until later, and the wafer will either have to be put through the photoresist stripping process again or discarded. Either alternative adds expense and time to the fabrication of the integrated circuit. 
     Another problem with the prior art method shown in  FIG. 1  is that the time of the photoresist stripping process is predetermined. As such, the time may not be optimally efficient. The time may be too short, in which case some or all of the wafers fail to be completely stripped of photoresist, requiring further processing of the wafers. Alternatively, the time may be too long, in which case the process continues after all the photoresist has been stripped from the wafer. This decreases throughput and fabrication efficiency. 
     It would therefore be desirable to provide a method and apparatus for detecting when essentially all the photoresist has been stripped from a wafer during the photoresist stripping process. Such a method and apparatus would preferably increase wafer fabrication throughput and efficiency. 
     SUMMARY OF THE INVENTION 
     Disclosed embodiments provide a method and apparatus for detecting the endpoint for a photoresist stripping process. Preferably, O and NO are introduced into the stripping chamber. When O and NO react, they produce NO 2  and emit light. However, while photoresist remains on the wafer, the O that is introduced mostly reacts with the photoresist and only a small amount of light is emitted from the O+NO reaction. After essentially all the photoresist has been stripped from the wafer, much of the O that would have reacted with the photoresist reacts with the NO instead. Thus, the rate of the O+NO reaction increases and the amount of light produced in the reaction increases. The method and apparatus detects the light emitted from the reaction of O and NO and uses the increase in the light emission levels to detect the endpoint of the photoresist stripping process. 
     According to a preferred embodiment, the apparatus includes a stripping chamber, a wafer disposed within the stripping chamber, and a light detecting apparatus for monitoring the intensity of light emitted within the stripping chamber. The light detecting apparatus detects the intensity of light emitted from the reaction of O and NO to form NO 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed embodiments will be better understood when consideration is given to the following detailed description in conjunction with the accompanying drawings, with like reference numerals designating like elements. 
         FIG. 1  is a flow chart of a prior art method for stripping photoresist. 
         FIG. 2  is a flow chart of a method for stripping photoresist. 
         FIG. 3  is a flow chart showing the process of photoresist stripping according to an embodiment of the present invention. 
         FIG. 4  is a schematic view of a first embodiment. 
         FIG. 5  is a schematic view of a second embodiment. 
         FIG. 6  is a schematic view of a third embodiment. 
         FIG. 7  is a graph showing the intensity of light detected from the reaction of O and NO. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows a flow chart depicting the major steps of a method  200  for detecting the endpoint of a photoresist stripping process. In an initial step  202 , the photoresist stripping process begins. The photoresist stripping process is detailed further in FIG.  3  and its accompanying description. 
     In a step  204 , an increase in emitted light is detected. Preferably, the light is emitted from the reaction of O and NO to form NO 2 . Suitable choice of an apparatus to detect the increase in emitted light, and suitable placement of the apparatus to detect the increase in emitted light are well within the capabilities of those skilled in the art of wafer processing. The increase in emitted light signals the endpoint of the photoresist stripping process. 
     The photoresist stripping process continues for a predetermined time in a step  206 . In a preferred embodiment, the predetermined time is twenty seconds. While the increase in light emitted signals that substantially all of the photoresist has been stripped from the wafer, the extra time for the photoresist stripping process ensures that the wafer has been sufficiently stripped of photoresist. 
     Finally, in a step  208 , the photoresist stripping process is ended. The photoresist has been stripped from the wafer and the wafer is ready for further processing. 
       FIG. 3  shows a flow chart depicting the photoresist stripping process  300  of a preferred embodiment. The photoresist stripping process  300  begins in a step  302 . As the process  300  begins, O and NO are introduced into the stripping chamber in a step  304 . In other embodiments, NO is not introduced into the chamber, but is introduced downstream from the stripping chamber outlet, into a flow of gas coming from the stripping chamber. In a preferred embodiment, the O and NO are produced by introducing O 2  and N 2  into a plasma chamber, dissociating the O 2  and N 2  within the plasma chamber so that a flow of O and NO forms and enters the stripping chamber, in such a manner that substantially no plasma enters the stripping chamber. 
     In a step  306 , the photoresist is stripped from a wafer. This occurs by a reaction of O with the photoresist, which removes the photoresist from the wafer. While the photoresist stripping occurs, most of the O introduced into the stripping chamber is consumed by the reaction with the photoresist. Only a small amount is available to combine with NO to create NO 2  and emit light. Thus, only a small amount of light is emitted. 
     In a step  308 , the photoresist stripping is completed. At this point substantially all of the photoresist has been removed from the wafer. Since the photoresist has been removed, the O no longer primarily reacts with the photoresist. As seen in a step  310 , the O is now available to combine with NO to create NO 2  and emit light. The increased availability of O increases the reaction of O and NO to create NO 2  and light. Thus, the intensity of emitted light increases. This increase in emitted light is a visible signal, as seen in a step  312 . The signal is observed and used to signal the endpoint of the photoresist stripping process in a step  314 . 
       FIG. 4  is a schematic view of a first embodiment of an apparatus  10 . A plasma chamber  12  comprises an inlet  14  and an outlet  16 . In a preferred embodiment, a flow of O 2  and N 2  enters the plasma chamber  12  through the plasma chamber inlet  14 . Within the plasma chamber  12 , the O 2  and N 2  are dissociated so that NO and O are formed. The flow of NO and O exits the plasma chamber  12  through the plasma chamber outlet  16 . A stripping chamber  18  comprises an inlet  20  and an outlet  22 . The plasma chamber outlet  16  is in fluid communication with the stripping chamber inlet  20 . Thus, the flow of NO and O enters the stripping chamber  18  as it leaves the plasma chamber  12 . Substantially no plasma enters the stripping chamber  18  from the plasma chamber  12 , only uncharged gas. 
     A wafer  24 , at least partially coated with a layer of photoresist  26 , is disposed inside the stripping chamber  18 . As the flow of O and NO passes through the stripping chamber  18 , the O reacts with the layer of photoresist  26  and removes the layer of photoresist  26  from the wafer  24 . Inside the stripping chamber  18 , O reacts with NO to form NO 2  and emit light. However, while the layer of photoresist  26  remains on the wafer  24 , much of the O is consumed by reacting with the layer of photoresist  26 . Little O is left over to react with NO, so little light is emitted. When the layer of photoresist  26  is substantially entirely removed from the wafer  24 , the O is no longer consumed by a reaction with the layer of photoresist  26 . The O now reacts with NO. More O reacts with NO after the layer of photoresist  26  is removed, so more light is emitted from the reaction of O and NO to form NO 2 . Therefore, the amount of emitted light increases after the layer of photoresist  26  has been essentially entirely removed. 
     A detecting apparatus  28  detects the level of light emitted by the reaction of O and NO to form NO 2  and emit light. In a preferred embodiment, the detecting apparatus  28  detects the light through a window  30 . However, there are many ways to arrange the detecting apparatus  28  so that it can detect the emitted light. In a preferred embodiment, the light emissions from the reaction of O and NO to form NO 2  are summed over the wavelength range of 470-770 nm while detecting intensity levels. 
       FIG. 5  is a schematic view of a second embodiment of the apparatus  10 . In this embodiment, a flow of O 2  enters the plasma chamber  12  through the plasma chamber inlet  14 . Within the plasma chamber  12 , the O 2  is dissociated so that O is formed. The flow of O exits the plasma chamber  12  through the plasma chamber outlet  16 . The flow of O enters the stripping chamber  18  as it leaves the plasma chamber  12 . Substantially no plasma enters the stripping chamber  18  from the plasma chamber  12 , only uncharged gas. A separate input  40  to the stripping chamber  18  is provided to supply a flow of NO to the stripping chamber  18 . 
     In the embodiment shown in  FIG. 5 , the window  30  and detecting apparatus  28  are placed close to the NO input, to aid detection of the light emitted in the reaction of O and NO. The detecting apparatus  28  detects the intensity of light emitted by the reaction of O and NO to form NO 2 , just as in the embodiment shown in FIG.  4 . 
       FIG. 6  is a schematic view of a third embodiment of the apparatus  10 . In this embodiment, the reaction of NO and O to produce NO 2  and light does not occur within the stripping chamber  18 . In this embodiment, a flow of O 2  enters the plasma chamber  12  and is dissociated so that a flow of O enters the stripping chamber  18 , while substantially no plasma enters the stripping chamber  18 . Substantially no NO exists in the stripping chamber  18 , so the level of light emitted inside the stripping chamber  18  from the reaction of O and NO to form NO 2  is essentially zero. As a flow of O leaves the stripping chamber  18  through the stripping chamber outlet  22 , it enters a downstream channel  60 . A flow of NO in introduced into the downstream channel  60  through an inlet  62  into the downstream channel  60 . In the downstream channel  60 , the flow of NO and the flow of O react to produce NO 2  and light. This light in the downstream channel  60  is detected by the detecting apparatus  28 . Preferably, the detecting apparatus detects the light through a window  30  in the downstream channel  60 , although there are many ways to arrange the detecting apparatus to detect the emitted light. The window  30  and detecting apparatus  28  are preferably located close to the inlet  60  into the downstream channel  60  to aid in detecting the light emitted in the reaction of O and NO. 
       FIG. 7  is a graph  70  showing the intensity of light detected from the reaction of O and NO during a preferred embodiment of the photoresist stripping process. The intensity levels on the graph  70  represent the summation of the intensity of light over the range of 470-770 nm. At a first time  72 , the photoresist stripping process has not begun and the light detected is at a low level. At a time  74 , the photoresist stripping process begins, and a flow of O and NO enters the stripping chamber. Some of the O and NO reacts to form NO 2  and emit light. As can be seen in the graph  70 , the intensity of light detected increases after O and NO enter the chamber at a time  74  until the intensity reaches a higher level at a time  76 . From a time  76  to a later time  78 , much of the O reacts with the photoresist, and is not available to react with NO. At a time  78 , substantially no photoresist remains on the wafer, and the O is now free to react with the NO. Thus from a time  78  to a later time  80 , the intensity of light detected greatly increases, more than doubling in intensity. This increase signals the endpoint of the photoresist stripping at a time  80 . The photoresist stripping process and the flow of NO and O continues until a time  82  to ensure essentially all the photoresist has been stripped from the wafer. At a time  82 , the flow of O and NO into the stripping chamber ends, and the intensity of light detected returns to a low level. 
     Although only a few embodiments have been described in detail herein, it should be understood that the described method and apparatus may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the method and apparatus are not to be limited to the details given herein, but may be modified within the scope of the appended claims.