Abstract:
A method for ashing a resist on a wafer in a plasma reaction chamber comprises the steps of flowing a non-activated oxygen containing gas into the plasma reaction chamber immediately before loading the wafer to the plasma reaction chamber, and then carrying out a plasma ashing of the resist. In one of the preferred embodiments, after the reaction chamber was exposed to the atmosphere and then evacuated to vacuum, a mixed gas of oxygen (90% in volume) and water vapor (10% in volume) was flown into the reaction chamber with 1000 seem and 1 Torr for 5 min. and subsequently the ashing was carried out. The method prevents the ashing rate from decreasing with ashing time.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for plasma ashing of photoresist on semiconductor wafers, and more specifically relates to a method for ashing photoresist with oxygen pretreatment of a reaction chamber. 
     DESCRIPTION OF THE PRIOR ART 
     It has been known that adding water vapor to an oxygen plasma generally enhances an ashing rate, and more particularly in a down-stream type ashing, has additional favorable effects on ashing photoresist on semiconductor wafers so that essentially no chemical etching of the semiconductor wafers and no corrosion of aluminum interconnections occurs. Instability of the ashing rate is a drawback of the down-stream type ashing by a water vapor added oxygen plasma. The instability is considered to be related to adsorption and desorption processes of water molecules on an inside wall of the reaction chamber. FIG. 1 shows changes of the ashing rate from the initial value after the reaction chamber was exposed to the atmosphere for repair and maintenance, in spite of the fact that the chamber was evacuated to vacuum before ashing. Attempts have been made to stabilize the ashing rate, such as performing a pretreatment by an oxygen plasma followed by an ashing process, or controlling a surface temperature of the inside wall of the reaction chamber. However, such attempts have failed to stabilize the ashing rate. For instance, as shown in FIG. 2, a pretreatment by the oxygen plasma has indeed recovered the ashing rate temporally, but does not maintain it, because the surface temperature of the inside wall of the reaction chamber is raised by the oxygen plasma itself, which evntually decreases the ashing rate. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for ashing a resist in a plasma reaction chamber which comprises the step of keeping a non-activated oxygen containing gas flowing into the plasma reaction chamber immediately before carrying out plasma ashing of the resist. 
     It is another object of the present invention to provide a method for ashing a resist in a plasma reaction chamber which comprises the steps of evacuating the plasma reaction chamber, generating an oxygen plasma in the plasma generation chamber, flowing a non-activated oxygen containing gas into the plasma reaction chamber, and loading the wafer having the resist thereon into the plasma reaction chamber for ashing. 
     The phrase &#34;non-activated oxygen containing gas&#34; means that gas molecules are neither ionized nor in an excited state, but in the ground state in the gas. A typical example is a neutral oxygen molecule (O 2 ) excluding oxygen ions (such as O +2 ), a neutral reactive oxygen species (O*), or ozone (O 3 ). 
     The non-activated oxygen containing gas, as a practical matter, selected from the group consisting of an essentially pure oxygen gas, a mixed gas of oxygen and water vapor, essentially the same mixed gas as a gas used for an ashing process, and a mixed gas of oxygen and any of inert gases. The pretreatment of the plasma reaction chamber by the non-activated oxygen containing gas can stabilize the ashing rate even after the plasma reaction chamber is exposed to the atmosphere. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will be more apparent from the following description, when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a graph showing time-dependence of an ashing rate by a mixed gas of oxygen and water vapor with vacuum pretreatment for every ashing, according to a typical prior art procedure after the reaction chamber is exposed to the atmosphere. As shown therein the ashing rate decreases gradually with ashing time, 
     FIG. 2 shows time-dependence of an ashing rate with vacuum pretreatment for the ashing, according to a typical prior art process after the reaction chamber is exposed to the atmosphere. Pretreatment by oxygen plasma recovers the ashing rate, but only temporarily. 
     FIG. 3 is a schematic illustration of a plasma reaction system for ashing a photoresist on a wafer according to the present invention. First and second reaction chambers 2, 5 are connected to a wafer transfer chamber 4 by first and second vacuum tight valves 3, 6, respectively. 
     FIG. 4 shows a cross-sectional view of a down-stream type ashing apparatus according to the present invention. A plasma reaction chamber 22 is separated from a plasma generation chamber 24 by a shower-head 23 through which only neutral reactive species are allowed to reach a wafer 21 to be ashed. 
     FIG. 5 shows time-dependence of an ashing rate for the first embodiment according to the present invention. After the reaction chamber is exposed to the atmosphere, vacuum pretreatment is followed by a oxygen flow. The oxygen flow recovers the ashing rate gradually. 
     FIG. 6 shows time-dependence of an ashing rate for the second embodiment according to the present invention. After the reaction chamber is exposed to the atmosphere, vacuum pretreatment is followed by flowing a mixed gas of oxygen (90%) and water vapor (10%). Flow of the mixed gas recovers the ashing rate similarly to the oxygen flow. 
     FIG. 7 shows time-dependence of an ashing rate for the third embodiment according to the present invention. After the reaction chamber is exposed to the atmosphere, vacuum pretreatment is followed by an oxygen plasma and subsequently flowing an oxygen gas. The oxygen gas flow maintains the high initial ashing rate. 
     FIG. 8 shows time-dependence of an ashing rate for the fourth embodiment according to the present invention. After the reaction chamber is exposed to the atmosphere, vacuum pretreatment is followed by an oxygen plasma and subsequently by flowing a mixed gas of oxygen (90%) and water vapor (10%). The mixed gas flow maintains the high initial ashing rate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following embodiments, a down-stream type ashing apparatus was used, as shown in FIG. 4. A wafer 21 to be ashed was placed on a wafer stage 20 heated at temperatue of 220° C. A microwave power level of 1.5 Kw at 13.56 MHz was supplied to a plasma generation chamber 24 through a quartz glass window 25. A plasma reaction chamber 22 was separated from the plasma generation chamber 24 by a shower-head 23 through which only neutral reactive species were allowed to reach the wafer 21. The wafer 21 was set apart from the shower-head 23 at such a distance that ions or microwaves leaking into the plasma reaction chamber 22 decayed sufficiently. Ashing conditions and pretreatment of an oxygen flow to measure the ashing rate were the same throughout the embodiments as shown in TABLE 1 unless otherwise indicated. 
     In the first embodiment, the reaction chamber was evacuated down to a pressure of ˜10 -4  Torr (1 Torr=133.3224 Pa) after being exposed to the atmosphere, and the initial ashing by a mixed gas of oxygen and water vapor resulted in a decrease of the ashing rate, for the same reason as in the prior art, and then a neutral oxygen gas was flowed into the reaction chamber for five minutes with a flow-rate of 1000 sccm and a pressure of 1 Torr. As shown in FIG. 5, the ashing rate recovered gradually, with three sets of consecutive oxygen flows, each of which was immediately followed by an ashing operation to measure the ashing rate. Table 1 below shows the preferred operating conditions. 
     
                       TABLE 1______________________________________Ashing            Pretreatment______________________________________O.sub.2     900    sccm    O.sub.2                             1000  sccmH.sub.2 O   100    sccm    Pressure                             1.0   TorrPressure    1.0    Torr    Duration                             5     minutesμ-wave   1.5    KWtemperature 220° C.of stagetemperature 40˜50° C.of the inside wallAshing time 30     sec______________________________________ 
    
     In the second embodiment, after the decrease of the ashing rate, for the same reason as in the first embodiment, the process gas, of oxygen (90% in vol.) and water vapor (10% in vol.), was flowed for five minutes with a flow-rate of 900 sccm for oxygen and 100 sccm for water vapor (10%), and a pressure of 1 Torr before each ashing. 
     As shown in FIG. 6, the ashing rate recovered gradually for three consecutive sets of process gas flows, each of which was immediately followed by an ashing operation to measure the ashing rate. 
     In the third embodiment, the same series of operations were carried out up to the end of the 60 second oxygen plasma as shown in FIG. 2, and subsequently three consecutive sets of an oxygen flow and an ashing operations to measure the ashing rate were performed. The result shown in FIG. 7 indicates no decrease after the oxygen plasma, unlike in FIG. 2. 
     In the fourth embodiment, the oxygen in the third embodiment was simply replaced by an ashing process gas composed of oxygen (90% in vol.) and water vapor (10% in vol.). The result shown in FIG. 8 is essentially the same as that of the third embodiment. 
     In the fifth embodiment, FIG. 3 shows the plasma reaction system for the present embodiment comprising the first plasma reaction chamber 2, the second plasma reaction chamber 5, the first wafer transfer chamber 1, the second wafer transfer chamber 4, and the third wafer transfer chamber 7. Each of the wafer transfer chambers 1, 4 and 7 has two vacuum tight valves. The vacuum tight valves 8 and 9 are located between the first wafer transfer chamber 1 and respectively the atmosphere and the first plasma reaction chamber 2. The vacuum tight valves 3 and 6 are located between the second wafer transfer chamber 4 and respectively the first plasma reaction chamber 2 and the second plasma reaction chamber 5. The vacuum tight valves 10 and 11 are located between the third wafer transfer chamber 7 and, respectively, the second plasma reaction chamber 5 and the atmosphere, respectively. All of the chambers can be evacuated by the vacuum system. In operation, when a wafer came into the first wafer transfer chamber 1 from the atmosphere, the valve 8 was open and the valve 9 was closed. After the first wafer transfer chamber 1 was evacuated sufficiently, the wafer was transfered to the first plasma reaction chamber 2 with the valve 8 closed and the valve 9 open. This load-locked sequence of these valve operations prevented the air from entering the first plasma reaction chamber 2, where halide gases were used for etching metal layers on the wafer. After the metal layers were etched, to form interconnection patterns, by halide gases, the wafer was transfered to the wafer transfer chamber 4, with the valve 3 open and the valve 6 closed, and then to the second plasma reaction chamber 5 with the valve 3 closed and the valve 6 open. This sequence prevented the halide gases from entering the second plasma reaction chamber 5. It usually takes 180 sec for the wafer to be transfered from the first plasma reaction chamber 2 to the second plasma reaction chamber 5. According to the fifth embodiment of the present invention, immediately before the valve 6 was opened to transfer the wafer into the second plasma reaction chamber 5 from the second wafer transfer chamber 4, the ashing process gas was flowed into the second plasma reaction chamber 5 for 170 sec. After that, the wafer was transfered to the second plasma reaction chamber 5 for ashing. The ashed wafer was transfered to the third wafer transfer chamber 7 with the valve 10 open and the valve 11 closed, and then the wafer came out of the third wafer transfer chamber 7 to the atmosphere with the valve 10 closed and the valve 11 open, whereby the air was prevented from entering the second plasma reaction chamber 5. A remarkable feature in this embodiment is that decreasing the ashing rate was prevented similarly to the first embodiment, and that without changing the timing sequence of the processes from the conventional one. 
     In the sixth embodiment, after aluminum-copper-titanium layers on a wafer were successively etched to form interconnection patterns in the first plasma reaction chamber 2 of the plasma reaction system shown in FIG. 3, the wafer was transfered to the wafer transfer chamber 4 with the valve 3 open and the valve 6 closed, and then to the second plasma reaction chamber 5 with the valve 3 closed and the valve 6 open. Similarly to the fifth embodiment, the process gas was flowed in the second plasma reaction chamber 5 with a flow rate of 1000 sccm and 1.0 Torr for 170 sec immediately before the valve 6 was opened to transfer the wafer into the second plasma reaction chamber 5 from the wafer transfer chamber 4. After that, the wafer was transfered to the second plasma reaction chamber 5 for ashing. In this embodiment, the same result was obtained as in the fifth embodiment, that is, prevention from decreasing the ashing rate again was obtained as well as without changing the timing sequence of the processes. 
     The technique according to the present invention is capable of maintaining the highest productivity of wafers by avoiding instability of the ashing rate.