Abstract:
A plasma etch process for successively different layers, including an anti-reflection coating (ARC), an amorphous carbon layer (ACL) and a dielectric layer, with successively different etch chemistries is performed in a single plasma reactor chamber. A first transition step is performed after etching the ARC by replacing the fluorine-containing process gas used in the ARC etch step with an inert species process gas. A flush step is performed after etching the ACL by replacing the hydrogen-containing process gas used in the ACL etch step with argon gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/793,935, filed Apr. 21, 2006. 

   BACKGROUND OF THE INVENTION 
   The etching of hard dielectric thin films on an integrated circuit substrate, such as a boron silicate glass film or a phosphorous silicate glass film, is preferably done using a hard mask rather than a photoresist mask. Such a hard mask, which may be an amorphous carbon layer or ACL, is better able to withstand strong plasma etch chemistries required for efficient etching of a hard dielectric film. A typical device structure consists of a photoresist mask, an anti-reflection coating (ARC) underlying the photoresist mask, the ACL underlying the ARC, and the hard dielectric film underlying the ACL. Each of the layers under the photoresist mask is etched in sequence using different plasma chemistries. Prior efforts to perform each of these etch steps in a single reactor (to maximize productivity) have failed because such attempts resulted in loss of control over critical dimension (feature size) and the occurrence of significant particulate contamination. These problems have not been well-understood, and the industry has avoided them by performing at least some of the sequential etch steps referred to above (e.g., the etching of the ACL in a hydrogen-based plasma chemistry) in a separate dedicated reactor chamber. However, if all the foregoing etch steps could be performed in the same plasma reactor chamber, then productivity would be greatly increased. 
   We have discovered that one problem with using a hardmask such as an ACL arises from the fluorine-based chemistry that must be used to etch the ARC (typically a silicon oxy-nitride). This chemistry leaves a fluorine-containing residue within the chamber. If the different etch steps are performed in the same reactor chamber, we believe that this fluorine-containing residue subsequently reacts (during the next etch process) with hydrogen present in the hydrogen and nitrogen-based chemistry used to etch the ACL. We feel that the combination of the fluorine and hydrogen reacts with carbon in the ACL to create carbon-containing particulate contamination. 
   Another problem we have discovered is that the hydrogen present in the nitrogen+hydrogen process gases employed to etch the ACL leaves a hydrogen or hydrogen containing residue in the chamber (on chamber surfaces and possibly on the wafer itself) that remains after conclusion of this etch step. We believe that, during the subsequent etching of the hard dielectric layer, hydrogen from the residue combines with fluorine in the fluorine-containing plasma chemistry used in the hard dielectric layer etch step. This reduces the amount of fluorine available to participate in the etch reaction, slowing down the etch rate and reducing control over the critical dimension (feature size) in the hard dielectric layer. Typically, the critical dimension is reduced below the desired magnitude. 
   Therefore, there is a need for a process for etching a hard dielectric layer (such as boron- or phosphorus-silicate glass) using a hard mask (such as amorphous carbon) in which the steps for etching the different layers with different plasma chemistries is carried out in the same reactor chamber without loss of control over critical dimension or feature size and without producing particulate contamination. 
   SUMMARY OF THE INVENTION 
   A plasma-enhanced process is performed in a single plasma reactor chamber for etching a thin film layer on a workpiece, using a hard mask layer including an amorphous carbon layer (ACL) overlying the thin film layer and an anti-reflection coating (ARC) overlying the ACL. The process includes etching a pattern in the ARC in accordance with a photoresist mask overlying the ARC, using a plasma produced from a fluorine-containing process gas, and then removing fluorine-containing residue from the reactor chamber and/or workpiece by performing a first transition step by replacing the fluorine-containing process gas with an inert species process gas and maintaining a plasma in the reactor chamber. A pattern is then etched in the ACL using the ARC as an etch mask by replacing the argon process gas with a process gas containing hydrogen while maintaining a plasma in the chamber. Thereafter, hydrogen-containing residue is removed from the reactor and/or from the chamber by performing a flush step by replacing the hydrogen-containing process gas with argon gas and maintaining a plasma in the chamber. The process continues with etching a pattern in the thin film layer using the ACL as a hard mask by replacing the argon gas in the chamber with a species capable of etching the thin film layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block flow diagram of a process embodying the invention. 
       FIGS. 2A ,  2 B,  2 C,  2 D,  2 E,  2 F and  2 G depict in chronological order the changes in a thin film structure during successive steps of the process of  FIG. 1 . 
       FIG. 3  depicts a plasma reactor of the type suited for carrying out the process of  FIG. 1  and having a controller programmed to execute the process of  FIG. 1 . 
       FIG. 4  is a graph depicting radial distributions of the oxide etch rate (in the dielectric thin film layer  10  of  FIG. 2 ) obtained in the ex-situ process (square symbols), the in-situ process (diamond symbols) and the in-situ+flush process of the present invention (triangle symbols). 
       FIG. 5  is a graph depicting radial distributions of the oxide etch selectivity (in the dielectric thin film layer  10  of  FIG. 2 ) using the same symbology used in  FIG. 4 . 
       FIG. 6  is a graph depicting the radial distributions of the top critical dimension obtained in the three processes, using the same symbology employed in the preceding graphs. 
       FIG. 7  is a graph depicting radial distributions of oxide etch depth obtained in the same three processes, using the symbology employed in the preceding graphs. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention concerns integrated circuit fabrication and specifically a plasma-enhanced etch process (illustrated in  FIG. 1 ) that is suitable for etching a hard dielectric thin film on a semiconductor wafer, such as boron-silicate glass (BSG) or phosphorus-silicate glass (PSG), using a hard mask layer such as an amorphous carbon layer (ACL). Typically, the amorphous carbon mask is etched in accordance with a desired mask pattern prior to etching the underlying hard dielectric thin film. For this purpose, an anti-reflection coating (ARC) is placed on the hard mask layer and a photoresist mask is placed on the ARC. The thin film structure at this point is depicted in  FIG. 2A , and includes a substrate or base layer  5  of a semiconductor wafer on which is formed a hard dielectric thin film  10  such as BSG or PSG. A hard mask  15  such as an amorphous carbon layer (ACL) overlies the hard dielectric thin film  10 . An anti-reflection coating (ARC)  20  of a material such as silicon oxy-nitride overlies the ACL  15 . Photoresist  25  is deposited on the ARC  20 . In the step of block  30  of  FIG. 1 , the photoresist  25  is formed into a mask by photolithographically forming apertures  32  through the resist  25  in accordance with a desired pattern. The wafer is then placed in a plasma etch reactor chamber (block  40  of  FIG. 1 ). The resist  25  is then employed as a mask in an etch step (block  45  of  FIG. 1 ) that extends the aperture  32  through the ARC  20 . The etch step of block  42  is carried out by introducing into the reactor chamber a process gas that (preferably) contains both a fluorocarbon gas and a fluoro-hydrocarbon gas and maintaining a chamber pressure between about 50 mT and 150 mT. One example of such a process gas is CF4 and CH2F2. Plasma source power and plasma bias power are coupled into the chamber. These conditions are maintained until the aperture  32  extends through the ARC  20  as shown in  FIG. 2C . 
   The next etch step to be performed in the reactor chamber extends the aperture  32  through the ACL  15  using the ARC  20  as the etch mask. However, before this next etch step is performed, a transition process step (block  44  of  FIG. 1 ) is performed first. One purpose of the transition step of block  44  is to stabilize the plasma in the chamber while the fluorine-based process gas is removed from the reactor chamber and the chamber pressure is changed and a new process gas flow rate is established. This is accomplished using an argon plasma chemistry, as depicted in  FIG. 2D . A more important purpose of the transition step of block  44  is to remove the fluorine-containing residue remaining the reactor chamber (e.g., on chamber interior surfaces or even on the wafer itself). The argon plasma chemistry is adequate to remove all the fluorine-containing residue. For this purpose, a very high argon gas flow rate (e.g., 800 sccm) is employed at a moderate chamber pressure (e.g., 100 mT). This process step preferably employs only modest plasma RF source power (100 Watts) and modest plasma RF bias power (e.g., 100 Watts). The modest amounts of source and bias power minimize (or prevent) undue ion bombardment effects on the wafer or thin film structure during the transition process step of block  44  of  FIG. 1 . 
   Upon removal of all the fluorine-containing residue, an etch step (block  46  of  FIG. 1 ) is performed that extends the opening  32  through the ACL  15 , as depicted in  FIG. 2E . This step is performed by flowing into the reactor chamber a process gas containing (preferably) hydrogen gas, nitrogen gas and carbon monoxide gas. In addition, the process gas can include at least one of the following gases: oxygen, methane (CH4), C2H4, sulfur dioxide (SO2), carbon dioxide (CO2). These gases react (or are precursors for species that react) with the photoresist layer  25  and remove it, as indicated in  FIG. 2E . The flow of argon is halted so that the argon gas is removed from the reactor chamber. This step leaves a hydrogen-containing residue in the reactor chamber (e.g., on chamber interior surfaces and possibly on the wafer itself). A flush step (block  48  of  FIG. 1 ) is performed to remove the hydrogen-containing residue. The flush step of block  48  employs an argon plasma with modest RF plasma source power (e.g., 100 Watts) and modest RF plasma bias power (e.g., 100 Watts) to avoid undue ion bombardment effects on the wafer. However, to ensure efficient removal of the hydrogen residue, this step employs a very high argon gas flow rate (e.g., 1000 sccm) at a modest chamber pressure (e.g., about 100 mT). The flush step of block  48  is depicted in  FIG. 2F . 
   The next etch step to be performed etches through the hard dielectric thin film using the ACL as a hard mask. However, before this etch step is performed, a transition step (block  50  of  FIG. 1 ) is performed which stabilizes the plasma while the plasma process conditions are changed to conform with the requirements of the etch step to be performed. This transition step (block  50 ) is performed using the argon plasma, but the gas (argon) flow rate and the chamber pressure are reduced to match the etch process gas flow rate (e.g., 274 sccm) and chamber pressure (e.g., 8-20 mT) that will be used during the etch step that follows. Also, the RF plasma source power is increased for the same purpose (e.g., from 100 Watts to 200 Watts). Once the plasma has stabilized, the hard dielectric thin film ( 10  of  FIG. 2A ) is etched (block  52  of  FIG. 1 ) to extend the opening  32  through the dielectric thin film  10 , as depicted in  FIG. 2G . Preferably the hard dielectric etch step of block  52  is performed with a process gas that includes C4F6, O2, Ar, CO and CH2F2 gases at a total gas flow rate of about 274 sccm at a chamber pressure of about 8 mT. RF plasma source power of 200 to 600 Watts is applied. RF bias power having components at an LF (2 MHz) frequency and an HF (13.56 MHz) frequency is applied to the wafer at a total power level of about 5000 Watts. 
     FIG. 3  depicts a plasma reactor for carrying out the process of  FIG. 1 . The reactor of  FIG. 3  has a vacuum chamber  100  defined by a cylindrical side wall  105  and a disk-shaped ceiling electrode  110 . An electrostatic chuck  115  supporting the wafer  120  includes an insulator layer  125  having an internal electrode  130  coupled to a D.C. chucking voltage supply  135 . An RF plasma bias power generator  140  is coupled to the electrode  130  through an impedance match element  145  and an isolation capacitor  150 . A VHF (e.g., 162 MHz) RF source power generator  155  is coupled through an impedance match element  162  to the ceiling electrode  110 . The impedance match element may be a coaxial tuning stub that is resonant near the frequency of the VHF generator  155 , the electrode  110  having a reactance that forms a resonance with plasma in the chamber  100  near the frequency of the VHF generator  155 , as disclosed in U.S. Pat. No. 6,528,751 issued Mar. 4, 2003 entitled PLASMA REACTOR WITH OVERHEAD RF ELECTRODE TUNED TO THE PLASMA by Daniel Hoffman et al., the disclosure of which is incorporated herein by reference. The ceiling electrode  110  is also a gas distribution showerhead. Process gases flow into the chamber  100  through an inner zone  160  of gas distribution orifices in the ceiling electrode  110  and an outer zone  165  of gas distribution orifices. A gas panel  170  provides selected process gases and apportions the gas flow between the inner and outer zones  160 ,  165 . Inner and outer electromagnets  180 ,  185  control plasma ion density distribution within the chamber  100  in accordance with D.C. currents applied to them from a D.C. current supply  190 . A vacuum pump  200  modulated by a throttle valve  205  establishes a desired vacuum chamber pressure in the chamber  100 . A master controller  195  is programmed with a set of instructions  196  to perform a process sequence of the type depicted in  FIG. 1 , and for that purpose controls the operation of the gas panel  170 , the D.C. magnet current supply  190 , the VHF source power generator  155 , the RF bias power generator  140  and the throttle valve  205 . In this case, the set  196  of instructions with which the master controller  195  is programmed are as follows: (1) control the gas panel  170  to flow CF4 and CH2F2 into the chamber  100  and control the source and bias power generators  155 ,  140  to apply RF plasma source power and RF plasma bias power (block  42  of  FIG. 1 ); (2) control the gas panel  170  to flow argon into the chamber  100  while maintaining a plasma and stop the flow of the fluorine-containing gases (block  44  of  FIG. 1 ); (3) control the gas panel  170  to flow nitrogen, hydrogen and carbon monoxide into the chamber  100  while maintaining the plasma and stop the flow of argon (block  46  of  FIG. 1 ); (4) control the gas panel  170  to flow argon at a high flow rate while maintaining a plasma and stop the flow of the N2, H2 and CO gases (block  48  of  FIG. 1 ); (5) reduce the flow rate of argon and increase the plasma source power (block  50  of  FIG. 1 ); and flow C4F6, O2, Ar, CO and CH2F2 while increasing the plasma source power and plasma bias power to high levels (block  52  of  FIG. 1 ). 
   The step of block  42  is preferably performed at a chamber pressure between 50 and 150 mT. 
   The step of block  44  is preferably performed with an argon gas flow rate of about 800 sccm at a chamber pressure between about 50 and 150 mT, with the throttle valve  205  100% open, 100 Watts of RF plasma source power from the generator  155 , 100 Watts of RF plasma bias power from the generator  140 , a ratio between the gas flow rates of the inner and outer gas orifice zones of 2:1 established by the gas panel  170 , 4 amperes D.C. driving the inner coil  180  and 4 amperes D.C. driving the outer coil  185 , this step being performed for 10 seconds. 
   The step of block  46  is preferably performed at a chamber pressure of about 100 mT. 
   The step of block  48  is performed with an argon gas flow rate of 1000 sccm established with the gas panel  170  at a chamber pressure of about 100 mT established by the throttle valve  205 , with 100 Watts of RF plasma source power from the generator  155 , 100 Watts of RF plasma bias power from the generator  140 , a ratio of 2.5 between the gas flow rates to the outer and inner orifice zones  165 ,  160 , and 6 amperes D.C. applied to the inner electromagnetic coil  180  only, for a time of about 15 seconds. 
   The step of block  50  is performed with a reduced argon gas flow rate of 274 sccm at a pressure of 8 mT, with 200 Watts of RF plasma source power, 100 Watts of RF plasma bias power and the same inner and outer zone gas flow ratio and electromagnetic coil current conditions as the previous step, for a time of about 10 seconds. 
   The etch step of block  52  is preferably performed with a process gas of C4F6, O2, Ar, CO, and CH2F2 at a flow rate of 274 sccm at a chamber pressure of 8 mT, 5000 Watts of RF plasma bias power with frequency components at 2 MHz and 13.56 MHz, and RF plasma source power between 200 and 600 Watts at a VHF frequency of 162 MHz. 
   Results: 
   We performed three etch processes on three wafers having the same thin film structure illustrated in  FIG. 2A . A first etch process (performed on a first wafer) was the prior art ex-situ process in which the different etch steps are performed in different plasma etch reactor chambers, or at least the fluorine chemistry etch steps (blocks  42  and  52  of  FIG. 1 ) were performed in a chamber different from the chamber in which the hydrogen chemistry etch step (block  46  of  FIG. 1 ) was performed. A second etch process (performed on a second wafer) was an in-situ process in which all the etch steps were performed in succession in the same plasma etch reactor chamber. The third etch process performed (on the third wafer) was the process of the present invention depicted in  FIG. 1 , which is an in-situ+flush process (i.e., with added flush and transitions steps between etch steps). Comparing the etch performance among the three wafers, the results were surprising. 
     FIG. 4  is a graph depicting radial distributions of the oxide etch rate (in the dielectric thin film layer  10  of  FIG. 2 ) obtained in the ex-situ process (square symbols), the in-situ process (diamond symbols) and the in-situ+flush process of the present invention (triangle symbols).  FIG. 4  shows that the in-situ process has inferior oxide etch rate compared with the ex-situ process, attributable to the presence of hydrogen that reduces fluorine species available for etch. Remarkably, the process of the invention nearly matches that of the ex-situ process except near the wafer periphery, this latter exception obtaining due to the improved uniformity of the process of the present invention and relative non-uniformity of the prior art ex-situ process. Thus, the present invention represents an improvement in oxide etch rate over the in-situ process and in uniformity of the radial distribution of the oxide etch rate over the ex-situ process. 
     FIG. 5  is a graph depicting radial distributions of the oxide etch selectivity (in the dielectric thin film layer  10  of  FIG. 2 ) using the same symbology used in  FIG. 4 .  FIG. 5  shows that the in-situ process has superior etch selectivity relative to the ex-situ process, which is to be expected because the presence of hydrogen reduces the fluorine availability, raising the proportion of carbon-to-fluorine in the polymer deposited on the mask during etch, thereby enhancing selectivity. Thus, it would be expected that in the present invention, which removes hydrogen to increase available fluorine, the etch selectivity would fall. However,  FIG. 5  indicates that quite the opposite occurs: etch selectivity in the process of the present invention is superior to the ex-situ process AND to the in-situ process. Therefore, the surprising result is that there is no trade-off in the present invention between etch selectivity and etch rate. 
     FIG. 6  is a graph depicting the radial distributions of the top critical dimension obtained in the three processes, using the same symbology employed in the preceding graphs.  FIG. 6  shows that the in-situ process yields the worst results while the present invention yields results as good as or nearly as good as the ex-situ process except near the wafer periphery. This last exception is attributable to the superior uniformity of results obtained in the process of the present invention and the relative non-uniformity of results obtained in the ex-situ process. 
     FIG. 7  is a graph depicting radial distributions of oxide etch depth obtained in the same three processes, using the symbology employed in the preceding graphs.  FIG. 7  shows that the present invention, while not yielding etch depths less than either of the other two processes, provides the most radially uniform distribution of oxide etch depth. 
   While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.