METHOD AND APPARATUS FOR DETECTING THE ENDPOINT OF A PHOTORESIST STRIPPING PROCESS

Abstract: A method and apparatus for detecting the endpoint of a photoresist stripping process. A wafer to be stripped of photoresist is disposed inside a stripping chamber. 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. A light detecting apparatus detects this increase in light intensity, which signals the endpoint of the photoresist stripping process.

Description: METHOD AND APPARATUS FOR DETECTING THE ENDPOINT OF PHOTORESIST STRIPPING PROCESS BACKGROUND OF THE INVENTION This disclosure relates generally to photoresist stπpping processes, and more particularly to a method and apparatus for detecting the endpoint of a photoresist stπpping process Fabπcation of integrated circuits generally starts with a thm slice of silicon called a wafer On this wafer one can fabπcate several hundred individual chips Each chip contain 10 to 20,000 components for a total of several million devices per wafer The devices include resistors, diodes, capacitors and transistors of \ aπous types The fabπcation of the devices includes depositing desired mateπals (such as silicon dioxide, aluminum, etc ) at certain locations A technique called photolithography is used to facilitate the introduction of mateπals at desired locations on the wafer and the removal of undesired mateπal at other locations As an example, a layer of aluminum is first deposited on the wafer Next, the wafer is coated \\ ith a light sensitive polymer called photoresist A mask is used to expose selected areas of photoresist to UV light The UV light induces polymeπzation m the exposed photoresist UV light causes the exposed photoresist to cross link, rendeπng 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 \\ afer with patterned photoresist is aluminum etched to remove the aluminum where there is no pattern This has the effect of transferπng the pattern to the aluminum, creating electπcal 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 stπpping The stπpping of photoresist from the w afer surface must be essentially complete, since photoresist left on the wafer surface will cause defects in the integrated circuit An important consideration m the photoresist stπpping 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 water Preferably, the time will not exceed the time when the photoresist has been entirely removed since this decreases the efficiency of the fabπcation of integrated circuits Figure 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 Figure 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 Figure 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 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. Figure 1 is a flow chart of a prior art method for stripping photoresist. Figure 2 is a flow chart of a method for stripping photoresist. Figure 3 is a flow chart showing the process of photoresist stripping according to an embodiment of the present invention. Figure 4 is a schematic view of a first embodiment. Figure 5 is a schematic view of a second embodiment. Figure 6 is a schematic view of a third embodiment. Figure 7 is a graph showing the intensity of light detected from the reaction of O and NO. DETAILED DESCRIPTION OF THE PREFERED EMBODIMENTS Figure 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 Figure 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?. 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. Figure 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 and N into a plasma chamber, dissociating the O 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 N0 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. Figure 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 and emit light. In a preferred embodiment, the detecting apparatus 28 detects the light through a window 30. However, there are many ways to aπange the detecting apparatus 28 so that it can detect the emitted light. In a prefeπed embodiment, the light emissions from the reaction of O and NO to form N0 are summed over the wavelength range of 470-770 nm while detecting intensity levels. Figure 5 is a schematic view of a second embodiment of the apparatus 10 In this embodiment, a flow of 0 2 enters the plasma chamber 12 through the plasma chamber mlet 14 Within the plasma chamber 12, the O? 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 stπpping chamber 18 as it leaves the plasma chamber 12 Substantially no plasma enters the stπpping chamber 18 from the plasma chamber 12, only uncharged gas A separate input 40 to the stπpping chamber 18 is provided to supply a flow of NO to the stπpping chamber 18 In the embodiment shown in Figure 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 N0 2 , just as in the embodiment shown in Figure 4 Figure 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? and light does not occur within the stπpping chamber 18 In this embodiment, a flow of 0 2 enters the plasma chamber 12 and is dissociated so that a flow of O enters the stπpping chamber 18, while substantially no plasma enters the stπpping chamber 18 Substantially no NO exists m the stπpping chamber 18, so the level of light emitted inside the stπpping chamber 18 from the reaction of O and NO to form N0 2 is essentially zero As a flow of O leaves the stπpping chamber 18 through the stπpping 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 N0 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 aπange the detecting apparatus to detect the emitted light The window 30 and detecting apparatus 28 are preferably located close to the mlet 60 into the downstream channel 60 to aid m detecting the light emitted in the reaction of O and NO Figure 7 is a graph 70 sho ing the intensity of light detected from the reaction of O and NO duπng a prefeπed embodiment of the photoresist stπpping process The intensity levels on the graph 70 represent the summation of the intensity of light o\ er the range of 470-770 nm At a first time 72, the photoresist stπpping process has not begun and the light detected is at a low level At a time 74, the photoresist stπpping process begins, and a flow of O and NO enters the stπpping chamber Some of the O and NO reacts to form N0 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.

Claims: CLAIMS What is claimed is: 1. A method for detecting an endpoint of a photoresist stripping process comprising the steps of: stripping photoresist from a wafer disposed inside a chamber; detecting an intensity of light emitted; and using a change in the intensity of light emitted to detect the endpoint of the photoresist stripping process. 2. The method of claim 1, wherein the chamber contains substantially no plasma. 3. The method of claim 1, further comprising the step of introducing a flow of O and a flow of NO into the chamber. 4. The method of claim 3 wherein the emitted light is emitted in a reaction of NO and O to form NO 2 . 5. The method of claim 3, wherein the O reacts with the photoresist to strip the photoresist from the wafer. 6. The method of claim 3 wherein the step of detecting a change in the intensity of light emitted further comprises detecting an increase in the intensity of light emitted from a reaction of NO and O. 7. The method of claim 6, wherein the increase in intensity of the light emitted from the reaction of NO and O signals the endpoint of the photoresist stripping process. 8. The method of claim 6, wherein the emitted light detected is in the wavelength range of about 470 nm to about 770 nm. 9. The method of claim 6, wherein a wall of the chamber includes a window, the intensity of the light emitted being detected through the window by a light detecting apparatus. 10. The method of claim 1, further comprising the steps of: introducing a flow of O into the chamber; providing an outlet from the chamber; providing a downstream channel, the downstream channel being in fluid communication with the chamber outlet; introducing a flow of NO into the downstream channel; detecting an increase in intensity of the light emitted from a reaction of NO and O, wherein the increase in intensity of the light emitted from the reaction of NO and O signals the endpoint of the photoresist stripping process. 11. The method of claim 10, wherein a wall of the downstream channel includes a window, the intensity of the light emitted being detected through the window by a light detecting apparatus. 12. A method for monitoring a photoresist stripping process, comprising the steps of: providing a stripping chamber with a first inlet and an outlet; providing a wafer at least partially coated with a photoresist layer; providing a detecting apparatus operable to detect an intensity of light emitted from a reaction of NO and O to form N0 2 ; disposing the wafer within the stripping chamber; introducing a flow of O into the stripping chamber through the first inlet, wherein the O reacts with the photoresist layer to strip the photoresist layer from the wafer; and detecting a change in the intensity of light emitted from the reaction of NO and O to form N0 2 with the detecting apparatus; and using the change in intensity of light emitted from the reaction of NO and O to form N0 2 to monitor the photoresist stripping process. 13. The method of claim 12, further comprising the step of introducing a flow of NO into the chamber through the first inlet, wherein an increase in the intensity of light emitted from the reaction of NO and O to form N0 2 signals the endpoint of the photoresist stripping process. 14. The method of claim 13, wherein a wall of the chamber includes a window, the intensity of the light emitted being detected through the window by a light detecting apparatus. 15. The method of claim 12, further comprising the step of introducing a flow of NO into the chamber through a second inlet, wherein an increase in the intensity of light emitted from the reaction of NO and O to form N0 2 signals the endpoint of the photoresist stripping process. 16. The method of claim 15, wherein a wall of chamber includes a window near the second inlet, the intensity of the light emitted being detected through the window by a light detecting apparatus. 17. The method of claim 12, further comprising the step of introducing a flow of NO into a downstream channel in fluid communication with the chamber outlet and wherein an increase in the intensity of light emitted from the reaction of NO and O to form N0 2 signals the endpoint of the photoresist stripping process. 18. The method of claim 17, wherein a wall of the downstream channel includes a window, the intensity of the light emitted being detected through the window by a light detecting apparatus. 19. An apparatus for detecting the endpoint of a photoresist stripping process, comprising: a stripping chamber; a wafer disposed within the stripping chamber; and a detecting apparatus operable to detect an intensity of light emitted. 20. The apparatus as recited in claim 19, wherein the detected light emitted is emitted by a reaction of O and NO to form N0 2 . 21. The apparatus as recited in claim 20, wherein a flow of NO and O is introduced into the stripping chamber through a first inlet. 22. The apparatus as recited by claim 21, wherein when substantially all of the photoresist is removed from the wafer, the rate of the reaction of O with NO to form NO? increases, with an accompanying increase in the amount of light emitted from the reaction of O with NO to form N0 2 . 23. The apparatus as recited by claim 22, further comprising: a plasma chamber having an inlet and an outlet, the plasma chamber outlet being in fluid communication with said stripping chamber first inlet, wherein a flow of 0 2 and N 2 is introduced into the plasma chamber inlet, the flow of 0 2 and N 2 is dissociated within the plasma chamber into O and NO, said O and NO leaving the plasma chamber through the plasma chamber outlet and entering the stripping chamber through the first stripping chamber inlet, such that substantially no plasma enters the stripping chamber. 24. The apparatus as recited by claim 23, further comprising a window in a wall of the stripping chamber, the detecting apparatus being outside the stripping chamber and detecting the light intensity through the window. 25. The apparatus as recited by claim 19, further comprising: a first inlet to the stripping chamber; a second inlet to the stripping chamber; a plasma chamber having an inlet and an outlet, the plasma chamber outlet being in fluid communication with the stripping chamber first mlet, wherein a flow of 0 2 is introduced into the plasma chamber inlet, the flow of 0 2 is dissociated within the plasma chamber into O, said O leaving the plasma chamber through the plasma chamber outlet and entering the stripping chamber through the stripping chamber first inlet, such that substantially no plasma enters the stripping chamber; and wherein a flow of NO is introduced into the stripping chamber through the stripping chamber second inlet, and when substantially all of the photoresist is removed from the wafer, the rate of the reaction of O with NO to form N0 increases, with an accompanying increase in the amount of light emitted from the reaction of O with NO to form N0 2 . 26. The apparatus as recited by claim 25, further comprising a window in a wall of the stripping chamber near the second inlet, the detecting apparatus being outside the stripping chamber and detecting the light intensity through the window. 27. The apparatus as recited by claim 19, further comprising: an inlet to the stripping chamber; an outlet from the stripping chamber; a plasma chamber having an inlet and an outlet, the plasma chamber outlet being in fluid communication with the stripping chamber inlet, wherein a flow of O 2 is introduced into the plasma chamber inlet, the flow of O 2 is dissociated within the plasma chamber into O, said O leaving the plasma chamber through the plasma chamber outlet and entering the stripping chamber through the stripping chamber first inlet, such that substantially no plasma enters the stripping chamber; a downstream channel, the downstream channel being in fluid communication with the stripping chamber outlet; and wherein a flow of NO is introduced into the downstream channel, and when substantially all of the photoresist is removed from the wafer, the rate of the reaction of O with NO to form N0 2 increases, with an accompanying increase in the amount of light emitted from the reaction of O with NO to form N0 2 . 28. The apparatus as recited by claim 27, further comprising a window in a wall of the downstream channel, the detecting apparatus being outside the downstream channel and detecting the light intensity through the window.

Inventors: COLLISON WENLI, NI TUQIANG, COLLISON, WENLI, NI, TUQIANG
Applicants: COLLISON WENLI, LAM RES CORP, NI TUQIANG, COLLISON, WENLI, LAM RESEARCH CORPORATION, NI, TUQIANG
Application year: 2000
Main classification: H01L21
IPC codes: H01J37, H01L21