Patent Publication Number: US-7595005-B2

Title: Method and apparatus for ashing a substrate using carbon dioxide

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for removing residue from a substrate. 
     BACKGROUND OF THE INVENTION 
     During semiconductor processing, a (dry) plasma etch process can be utilized to remove or etch material along fine lines or within vias or contacts patterned on a silicon substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer, for example a photoresist layer, in a processing chamber. Once the substrate is positioned within the chamber, an ionizable, dissociative gas mixture is introduced within the chamber at a pre-specified flow rate, while a vacuum pump is throttled to achieve an ambient process pressure. Thereafter, a plasma is formed when a fraction of the gas species present are ionized by electrons heated via the transfer of radio frequency (RF) power either inductively or capacitively, or microwave power using, for example, electron cyclotron resonance (ECR). Moreover, the heated electrons serve to dissociate some species of the ambient gas species and create reactant specie(s) suitable for the exposed surface etch chemistry. Once the plasma is formed, selected surfaces of the substrate are etched by the plasma. The process is adjusted to achieve appropriate conditions, including an appropriate concentration of desirable reactant and ion populations to etch various features (e.g., trenches, vias, contacts, etc.) in the selected regions of the substrate. Such substrate materials where etching is required include silicon dioxide (SiO 2 ), low dielectric constant (i.e., low-k) dielectric materials, poly-silicon, and silicon nitride. Once the pattern is transferred from the patterned photoresist layer to the underlying dielectric layer, using, for example, dry plasma etching, the remaining layer of photoresist, and post-etch residues, are removed via an ashing (or stripping) process. For instance, in conventional ashing processes, the substrate having the remaining photoresist layer is exposed to an oxygen plasma formed from the introduction of diatomic oxygen (O 2 ) and ionization/dissociation thereof. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of removing residue on a substrate using a plasma ashing process. The plasma ashing process utilizes a process gas comprising carbon dioxide. Additionally, the process gas can comprise a passivation gas, such as a hydrocarbon gas. 
     According to one embodiment, a method for removing residue from a substrate and a computer readable medium with program instructions to cause a computer system to control a residue removal process is described, comprising: disposing the substrate in a plasma processing system, the substrate having a dielectric layer formed thereon and a mask layer overlying the dielectric layer, wherein the mask layer comprises a pattern formed therein and the dielectric layer comprises a feature formed therein as a result of an etching process used to transfer the pattern in the mask layer to the dielectric layer; introducing a process gas comprising carbon dioxide (CO 2 ); forming a plasma from the process gas in the plasma processing system; and removing the mask layer from the substrate with the plasma. 
     According to another embodiment, a plasma processing system for removing photoresist from a substrate is described, comprising: a plasma processing chamber for facilitating the formation of a plasma from a process gas; and a controller coupled to the plasma processing chamber and configured to execute a process recipe utilizing the process gas to form a plasma to remove the photoresist from the substrate, wherein the process gas comprises carbon dioxide (CO 2 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A ,  1 B, and  1 C show another schematic representation of a typical procedure for pattern etching a thin film; 
         FIG. 2  shows a simplified schematic diagram of a plasma processing system according to an embodiment of the invention; 
         FIG. 3  shows a schematic diagram of a plasma processing system according to another embodiment of the invention; 
         FIG. 4  shows a schematic diagram of a plasma processing system according to another embodiment of the invention; 
         FIG. 5  shows a schematic diagram of a plasma processing system according to another embodiment of the invention; 
         FIG. 6  shows a schematic diagram of a plasma processing system according to another embodiment of the invention; 
         FIG. 7  presents a method of removing a mask layer on a substrate in a plasma processing system according to an embodiment of the invention; and 
         FIG. 8  presents a method of etching a thin film on a substrate according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS 
     In the following description, for purposes of explanation and not limitation, specific details are set forth, such as a particular geometry of the plasma processing system and descriptions of various processes. However, it should be understood that the invention may be practiced in other embodiments that depart from these specific details. 
     In material processing methodologies, pattern etching comprises the application of a thin layer of light-sensitive material, such as photo-resist, to an upper surface of a substrate that is subsequently patterned in order to provide a mask for transferring this pattern to the underlying thin film on a substrate during etching. The patterning of the light-sensitive material generally involves exposure of the light-sensitive material to a geometric pattern of electro-magnetic (EM) radiation using, for example, a micro-lithography system, followed by the removal of the irradiated regions of the light-sensitive material (as in the case of positive photo-resist), or non-irradiated regions (as in the case of negative resist) using a developing solvent. Moreover, this mask layer may comprise multiple sub-layers. 
     For example, as shown in  FIGS. 1A through 1C , a mask comprising light-sensitive layer  3  with pattern  2  (such as patterned photoresist) can be utilized for transferring feature patterns into a thin film  4  on a substrate  5 . The pattern  2  is transferred to the thin film  4  using, for instance, dry plasma etching in order to form feature  6 . Upon completion of etching, the mask  3  is removed. 
     As noted above, the mask is conventionally removed by exposing the mask to a plasma formed from a gas containing O 2 . The present inventors have recognized, however, that such a method can damage dielectric films, and in particular low-k dielectric films. Such damage may be damage that affects the critical dimension of a feature etched in the dielectric, or damage that increases the dielectric constant of the dielectric. The inventors further recognized that carbon can passivate dielectric and low-k films to minimize damage to such films. Thus, in one embodiment, a process gas comprising carbon dioxide (CO 2 ) is utilized for removing mask  3 . The use of CO 2  can reduce damage to the dielectric when compared to O 2  plasma etching process. In another embodiment, a process gas comprising carbon dioxide (CO 2 ) and a passivation gas is utilized for removing mask  3 . For example, the passivation gas can include a hydrocarbon gas (C x H y ), wherein x, y represent integers greater than or equal to unity. The hydrocarbon gas (C x H y ) can include one or more of C 2 H 4 , CH 4 , C 2 H 2 , C 2 H 6 , C 3 H 4 , C 3 H 6 , C 3 H 8 , C 4 H 6 , C 4 H 8 , C 4 H 10 , C 5 H 8 , C 5 H 10 , C 6 H 6 , C 6 H 10 , or C 6 H 12 , or two or more thereof. Alternatively, the process gas can further comprise an inert gas, such as a Noble gas (i.e., He, Ne, Ar, Kr, Xe, Rn). Alternatively yet, the process gas can further comprise O 2 , CO, NO, NO 2 , or N 2 O, or a combination of two or more thereof. 
     According to one embodiment, a plasma processing system  1  is depicted in  FIG. 2  comprising a plasma processing chamber  10 , a diagnostic system  12  coupled to the plasma processing chamber  10 , and a controller  14  coupled to the diagnostic system  12  and the plasma processing chamber  10 . The controller  14  is configured to execute a process recipe comprising at least one of the above-identified chemistries (i.e. CO 2  with and without C x H y  and/or other gases, etc.) to remove photoresist and/or other residue, such as etch residue, from a substrate. Additionally, controller  14  is configured to receive at least one endpoint signal from the diagnostic system  12  and to post-process the at least one endpoint signal in order to accurately determine an endpoint for the process. In the illustrated embodiment, plasma processing system  1 , depicted in  FIG. 2 , utilizes plasma for material processing. Plasma processing system  1  can comprise an etch chamber, and ash chamber, or combination thereof. 
     According to the embodiment depicted in  FIG. 3 , plasma processing system  1   a  can comprise plasma processing chamber  10 , substrate holder  20 , upon which a substrate  25  to be processed is affixed, and vacuum pumping system  30 . Substrate  25  can be a semiconductor substrate, a wafer or a liquid crystal display. Plasma processing chamber  10  can be configured to facilitate the generation of plasma in processing region  15  adjacent a surface of substrate  25 . An ionizable gas or mixture of gases is introduced via a gas injection system (not shown) and the process pressure is adjusted. For example, a control mechanism (not shown) can be used to throttle the vacuum pumping system  30 . Plasma can be utilized to create materials specific to a pre-determined materials process, and/or to aid the removal of material from the exposed surfaces of substrate  25 . The plasma processing system  1   a  can be configured to process substrates of any desired size, such as 200 mm substrates, 300 mm substrates, or larger. 
     Substrate  25  can be affixed to the substrate holder  20  via an electrostatic clamping system Furthermore, substrate holder  20  can further include a cooling system including a re-circulating coolant flow that receives heat from substrate holder  20  and transfers heat to a heat exchanger system (not shown), or when heating, transfers heat from the heat exchanger system. Moreover, gas can be delivered to the back-side of substrate  25  via a backside gas system to improve the gas-gap thermal conductance between substrate  25  and substrate holder  20 . Such a system can be utilized when temperature control of the substrate is required at elevated or reduced temperatures. For example, the backside gas system can comprise a two-zone gas distribution system, wherein the helium gas gap pressure can be independently varied between the center and the edge of substrate  25 . In other embodiments, heating/cooling elements, such as resistive heating elements, or thermoelectric heaters/coolers can be included in the substrate holder  20 , as well as the chamber wall of the plasma processing chamber  10  and any other component within the plasma processing system  1   a.    
     In the embodiment shown in  FIG. 3 , substrate holder  20  can comprise an electrode through which RF power is coupled to the processing plasma in process space  15 . For example, substrate holder  20  can be electrically biased at a RF voltage via the transmission of RF power from a RF generator  40  through an impedance match network  50  to substrate holder  20 . The RF bias can serve to heat electrons to form and maintain plasma. In this configuration, the system can operate as a reactive ion etch (RIE) reactor, wherein the chamber and an upper gas injection electrode serve as ground surfaces. A typical frequency for the RF bias can range from about 0.1 MHz to about 100 MHz. RF systems for plasma processing are well known to those skilled in the art. 
     Alternately, RF power is applied to the substrate holder electrode at multiple frequencies. Furthermore, impedance match network  50  serves to improve the transfer of RF power to plasma in plasma processing chamber  10  by reducing the reflected power. Match network topologies (e.g. L-type, π-type, T-type, etc.) and automatic control methods are well known to those skilled in the art. 
     Furthermore, the plasma processing system  1   a  comprises a gas injection system (not shown) configured to introduce process gases to processing region  15 . The gas injection system may include a shower head gas injection system located above the substrate, which is well known to those skilled in the art of vacuum processing. Alternatively, the gas injection system may include a multi-zone gas injection system. For example, the gas injection system can include a first set of gas injection orifices coupled to a gas supply system and configured to introduce process gas to a substantially central region above the substrate, and a second set of gas injection orifices coupled to a gas supply system and configured to introduce process gas to a substantially peripheral region above the substrate. 
     Vacuum pump system  30  can include a turbo-molecular vacuum pump (TMP) capable of a pumping speed up to about 5000 liters per second (and greater) and a gate valve for throttling the chamber pressure. In conventional plasma processing devices utilized for dry plasma etch, a 1000 to 3000 liter per second TMP is generally employed. TMPs are useful for low pressure processing, typically less than about 50 mtorr. For high pressure processing (i.e., greater than about 100 mTorr), a mechanical booster pump and dry roughing pump can be used. Furthermore, a device for monitoring chamber pressure (not shown) can be coupled to the plasma processing chamber  10 . The pressure measuring device can be, for example, a Type 628B Baratron absolute capacitance manometer commercially available from MKS Instruments, Inc. (Andover, Mass.). 
     Controller  14  comprises a microprocessor, memory, and a digital I/O port capable of generating control voltages sufficient to communicate and activate inputs to plasma processing system  1   a  as well as monitor outputs from plasma processing system  1   a . Moreover, controller  14  can be coupled to and can exchange information with RF generator  40 , impedance match network  50 , the gas injection system (not shown), vacuum pump system  30 , as well as the backside gas delivery system (not shown), the substrate/substrate holder temperature measurement system (not shown), and/or the electrostatic clamping system (not shown). For example, a program stored in the memory can be utilized to activate the inputs to the aforementioned components of plasma processing system  1   a  according to a process recipe in order to perform the method of removing photoresist from a substrate. One example of controller  14  is a DELL PRECISION WORKSTATION 610™, available from Dell Corporation, Austin, Tex. 
     Controller  14  can be locally located relative to the plasma processing system  1   a , or it can be remotely located relative to the plasma processing system  1   a . For example, controller  14  can exchange data with plasma processing system  1   a  using at least one of a direct connection, an intranet, and the internet. Controller  14  can be coupled to an intranet at, for example, a customer site (i.e., a device maker, etc.), or it can be coupled to an intranet at, for example, a vendor site (i.e., an equipment manufacturer). Additionally, for example, controller  14  can be coupled to the internet. Furthermore, another computer (i.e., controller, server, etc.) can, for example, access controller  14  to exchange data via at least one of a direct connection, an intranet, and the internet. 
     The diagnostic system  12  can include an optical diagnostic subsystem (not shown). The optical diagnostic subsystem can comprise a detector such as a (silicon) photodiode or a photomultiplier tube (PMT) for measuring the light intensity emitted from the plasma. The diagnostic system  12  can further include an optical filter such as a narrow-band interference filter. In an alternate embodiment, the diagnostic system  12  can include at least one of a line CCD (charge coupled device), a CID (charge injection device) array, and a light dispersing device such as a grating or a prism. Additionally, diagnostic system  12  can include a monochromator (e.g., grating/detector system) for measuring light at a given wavelength, or a spectrometer (e.g., with a rotating grating) for measuring the light spectrum such as, for example, the device described in U.S. Pat. No. 5,888,337, the entire content of which is incorporated herein by reference. 
     The diagnostic system  12  can include a high resolution Optical Emission Spectroscopy (OES) sensor such as from Peak Sensor Systems, or Verity Instruments, Inc. Such an OES sensor has a broad spectrum that spans the ultraviolet (UV), visible (VIS), and near infrared (NIR) light spectrums. The resolution is approximately 1.4 Angstroms; that is, the sensor is capable of collecting 5550 wavelengths from 240 to 1000 nm. The OES sensor can be equipped with high sensitivity miniature fiber optic UV-VIS-NIR spectrometers which are, in turn, integrated with 2048 pixel linear CCD arrays. 
     The spectrometers receive light transmitted through single or bundled optical fibers, where the light output from the optical fibers is dispersed across the line CCD array using a fixed grating. Similar to the configuration described above, light emitting through an optical vacuum window is focused onto the input end of the optical fibers via a convex spherical lens. Three spectrometers, each specifically tuned for a given spectral range (UV, VIS and NIR), can form a sensor for a process chamber. Each spectrometer can include an independent A/D converter. And lastly, depending upon the sensor utilization, a full emission spectrum can be recorded every 0.1 to 1.0 seconds. 
     In the embodiment shown in  FIG. 4 , the plasma processing system  1   b  can be similar to the embodiment of  FIG. 3  and further comprise either a stationary, or mechanically or electrically rotating magnetic field system  60 , in order to potentially increase plasma density and/or improve plasma processing uniformity, in addition to those components described with reference to  FIG. 3 . Moreover, controller  14  can be coupled to magnetic field system  60  in order to regulate the speed of rotation and field strength. The design and implementation of a rotating magnetic field is well known to those skilled in the art. 
     In the embodiment shown in  FIG. 5 , the plasma processing system  1   c  can be similar to the embodiment of  FIG. 3  or  FIG. 4 , and can further comprise an upper electrode  70  to which RF power can be coupled from RF generator  72  through impedance match network  74 . A frequency for the application of RF power to the upper electrode can range from about 0.1 MHz to about 200 MHz. Additionally, a frequency for the application of power to the lower electrode can range from about 0.1 MHz to about 100 MHz. Moreover, controller  14  is coupled to RF generator  72  and impedance match network  74  in order to control the application of RF power to upper electrode  70 . The design and implementation of an upper electrode is well known to those skilled in the art. 
     In the embodiment shown in  FIG. 6 , the plasma processing system  1   d  can be similar to the embodiments of  FIGS. 3 ,  4  and  5 , and can further comprise an inductive coil  80  to which RF power is coupled via RF generator  82  through impedance match network  84 . RF power is inductively coupled from inductive coil  80  through a dielectric window (not shown) to plasma processing region  45 . A frequency for the application of RF power to the inductive coil  80  can range from about 10 MHz to about 100 MHz. Similarly, a frequency for the application of power to the chuck electrode can range from about 0.1 MHz to about 100 MHz. In addition, a slotted Faraday shield (not shown) can be employed to reduce capacitive coupling between the inductive coil  80  and plasma. Moreover, controller  14  is coupled to RF generator  82  and impedance match network  84  in order to control the application of power to inductive coil  80 . In an alternate embodiment, inductive coil  80  can be a “spiral” coil or “pancake” coil in communication with the plasma processing region  15  from above as in a transformer coupled plasma (TCP) reactor. The design and implementation of an inductively coupled plasma (ICP) source, or transformer coupled plasma (TCP) source, is well known to those skilled in the art. 
     Alternately, the plasma can be formed using electron cyclotron resonance (ECR). In yet another embodiment, the plasma is formed from the launching of a Helicon wave. In yet another embodiment, the plasma is formed from a propagating surface wave. Each plasma source described above is well known to those skilled in the art. 
     In the following discussion, a method of removing a mask layer or residue or both from a substrate utilizing a plasma processing device is presented. The plasma processing device can comprise various elements, such as described in  FIGS. 2 through 6 , and combinations thereof. 
     In one embodiment, the removal process comprises forming plasma from a process gas including carbon dioxide. In another embodiment, the process gas further includes a passivation gas, such as a hydrocarbon gas (C x H y ). For example, a process parameter space can comprise a chamber pressure of about 20 to about 1000 mTorr, a CO 2  process gas flow rate ranging from about 50 to about 1000 sccm, an optional C x H y  process gas flow rate ranging from about 50 to about 1000 sccm, an upper electrode (e.g., element  70  in  FIG. 5 ) RF bias ranging from about 500 to about 2000 W, and a lower electrode (e.g., element  20  in  FIG. 5 ) RF bias ranging from about 10 to about 500 W. Also, the upper electrode bias frequency can range from about 0.1 MHz to about 200 MHz, e.g., about 60 MHz. In addition, the lower electrode bias frequency can range from about 0.1 MHz to about 100 MHz, e.g., about 2 MHz. 
     In another alternate embodiment, RF power is supplied to the upper electrode and not the lower electrode. In another alternate embodiment, RF power is supplied to the lower electrode and not the upper electrode. 
     In general, the time to remove the mask layer or residue or both can be determined using design of experiment (DOE) techniques; however, it can also be determined using endpoint detection. One possible method of endpoint detection is to monitor a portion of the emitted light spectrum from the plasma region that indicates when a change in plasma chemistry occurs due to substantially near completion of the removal of photoresist from the substrate and contact with the underlying material film. For example, a portion of the spectrum that indicates such changes includes a wavelength of 482.5 nm (CO), and can be measured using optical emission spectroscopy (OES). After emission levels corresponding to the monitored wavelengths cross a specified threshold (e.g., drop to substantially zero or increase above a particular level), an endpoint can be considered to be complete. Other wavelengths that provide endpoint information can also be used. Furthermore, the etch time can be extended to include a period of over-ash, wherein the over-ash period constitutes a fraction (i.e. 1 to 100%) of the time between initiation of the etch process and the time associated with endpoint detection. 
     In an example, a method of removing a mask layer and post-etch residue following a dry etching process for transferring a pattern to an underlying dielectric layer is presented. The dielectric layer comprises an ultra-low-dielectric constant (ultra-low-k, or ULK) material. For example, the ULK material includes a porous SiCOH film formed using a plasma enhanced chemical vapor deposition (PECVD) process. The removal process may be performed utilizing a plasma processing device such as the one described in  FIG. 3 . However, the methods discussed are not to be limited in scope by this exemplary presentation. 
     As noted above, the present inventors discovered that using CO 2  in a plasma ashing process can reduce damage to the dielectric compared to an O 2  ashing process. The inventors have further discovered that varying certain aspects of the ashing process can reduce critical dimension shift for features etched in the dielectric. Table 1 presents the shift in critical dimensions (CD shift; measured in nanometers, m) of a feature etched in the ULK film, following various dry plasma ashing processes (presented in Table 1), and a HF wet clean. The CD shift is provided at the feature top (Top), feature mid-depth (Mid) and feature bottom (Bot) for features that are densely populated within the ULK film (dense ULK CD shift) and for features that are not densely populated within the ULK film (isolated (Iso) ULK CD shift). 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Ash Step 
                   
                 Dense ULK CD Shift 
                 Iso ULK CD Shift 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Pres- 
                 Bias 
                   
                 Process 
                 Center 
                 Edge 
                 Center 
                 Edge 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Coupon 
                 sure 
                 Power 
                 Process gas 
                 variations 
                 Top 
                 Mid 
                 Bot 
                 Top 
                 Mid 
                 Bot 
                 Top 
                 Mid 
                 Bot 
                 Top 
                 Mid 
                 Bot 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 C0 
                 100 mT 
                 600 W 
                 CO2 = 750 sccm 
                 No 
                 25 
                 9 
                 7 
                 — 
                 — 
                 — 
                 28 
                 9 
                 7 
                 34 
                 13 
                 10 
               
               
                 C1 
                 100 mT 
                 400 W 
                 CO2 = 750 sccm 
                 No 
                 40 
                 16 
                 6 
                 46 
                 18 
                 7 
                 45 
                 18 
                 7 
                 45 
                 18 
                 7 
               
               
                 C2 
                  75 mT 
                 400 W 
                 CO2 = 750 sccm 
                 No 
                 36 
                 13 
                 5 
                 38 
                 15 
                 6 
                 39 
                 16 
                 6 
                 38 
                 15 
                 5 
               
               
                 C3 
                 100 mT 
                 800 W 
                 CO2 = 750 sccm 
                 No 
                 21 
                 7 
                 5 
                 24 
                 10 
                 5 
                 23 
                 7 
                 5 
                 23 
                 11 
                 5 
               
               
                 C4 
                 100 mT 
                 600 W 
                 CO2 = 1000 sccm 
                 No 
                 26 
                 7 
                 5 
                 28 
                 9 
                 5 
                 26 
                 9 
                 6 
                 26 
                 9 
                 5 
               
               
                 C5 
                  80 mT 
                 800 W 
                 CO2 = 750 sccm, 
                 No 
                 15 
                 7 
                 5 
                 16 
                 7 
                 6 
                 15 
                 7 
                 5 
                 15 
                 8 
                 5 
               
               
                   
                   
                   
                 GR = 90 
               
               
                 C8 
                 100 mT 
                 600 W 
                 CO2/CH4 = 750/ 
                 No 
                 25 
                 8 
                 6 
                 21 
                 9 
                 6 
                 24 
                 10 
                 5 
                 22 
                 13 
                 5 
               
               
                   
                   
                   
                 100 sccm 
               
               
                 C15 
                 100 mT 
                 600 W 
                 CO2/N2 = 750/ 
                 No 
                 43 
                 17 
                 7 
                 49 
                 16 
                 7 
                 42 
                 16 
                 8 
                 45 
                 15 
                 8 
               
               
                   
                   
                   
                 100 sccm 
               
               
                 C9 
                  80 mT 
                 800 W 
                 CO2-750 sccm, GR90 
                 10 mT 
                 15 
                 5 
                 4 
                 19 
                 5 
                 4 
                 14 
                 5 
                 4 
                 19 
                 6 
                 5 
               
               
                   
                   
                   
                   
                 CH4/Ar 
               
               
                 C10 
                  80 mT 
                 800 W 
                 CO2-750 sccm, GR90 
                 15 mT 
                 14 
                 4 
                 4 
                 16 
                 4 
                 4 
                 15 
                 5 
                 5 
                 18 
                 5 
                 5 
               
               
                   
                   
                   
                   
                 CH4/Ar 
               
               
                 C12 
                  80 mT 
                 800 W 
                 CO2-750 sccm, GR90 
                 50 mT 
                 14 
                 4 
                 4 
                 17 
                 4 
                 4 
                 16 
                 5 
                 5 
                 18 
                 6 
                 5 
               
               
                   
                   
                   
                   
                 CH4/Ar 
               
               
                 C11 
                  80 mT 
                 800 W 
                 CO2-750 sccm, GR90 
                 50 mT 
                 15 
                 4 
                 4 
                 19 
                 4 
                 4 
                 15 
                 6 
                 5 
                 19 
                 6 
                 5 
               
               
                   
                   
                   
                   
                 CH4/N2 
               
               
                 C13 
                  80 mT 
                 800 W 
                 CO2-750 sccm, GR90 
                 50 mT 
                 17 
                 7 
                 5 
                 22 
                 7 
                 6 
                 18 
                 7 
                 6 
                 21 
                 6 
                 6 
               
               
                   
                   
                   
                   
                 CH4/Ar 
               
               
                   
                   
                   
                   
                 during ash 
               
               
                   
               
            
           
         
       
     
     As illustrated in Table 1, variations in the process conditions for the dry plasma ashing step are performed, including changes to the process pressure, bias RF power, and the process gas composition. From inspection of Table 1, an increase in the flow rate of CO 2  can lead to a marginal reduction in the CD shift for both dense and isolated features (see coupons C0 and C4). Additionally, a reduction in the process pressure can lead to a marginal reduction in the CD shift for both dense and isolated features (see coupons C1 and C2). Additionally yet, an increase in the bias RF power can lead to a marginal reduction in the CD shift for both dense and isolated features (see coupons C1 and C4). From further inspection of Table 1, the addition of a passivation gas including CH 4  to the process gas composition can lead to a marginal reduction in the CD shift for both dense and isolated features (see coupons C1 and C8). For coupon C5, 90% of the total flow of process gas is introduced above the substrate substantially near the center portion of the substrate, while the remaining 10% of the total flow rate of process gas is introduced substantially near the edge portion of the substrate. In doing so, the CD shift for both dense and isolated features is further reduced. 
     Referring still to Table 1, coupons C9 through C12 illustrate the effect of performing a pre-treatment step prior to the dry plasma ashing step. The pre-treatment step comprises exposing the substrate to plasma formed from a pre-treatment process gas having a hydrocarbon gas (such as CH 4 ) and an inert gas. The inert gas may include a noble gas, such as Ar, or it may include N 2 . The use of a pre-treatment step affects a marginal reduction in the CD shift in both dense and isolated features. For coupon C13, the introduction of a hydrocarbon gas and an inert gas (Ar) is performed during the dry plasma ashing step. The pre-treatment process marginally affects the CD shift following the residue removal process(es). 
       FIG. 7  presents a flow chart of a method for removing residue on a substrate in a plasma processing system according to an embodiment of the present invention. Procedure  400  begins in  410  in which a process gas is introduced to the plasma processing system, wherein the process gas comprises carbon dioxide (CO 2 ). The process gas can further comprise a passivation gas. For example, the passivation gas can include a hydrocarbon gas (C x H y ), wherein x, y represent integers greater than or equal to unity. The hydrocarbon gas (C x H y ) can include one or more of C 2 H 4 , CH 4 , C 2 H 2 , C 2 H 6 , C 3 H 4 , C 3 H 6 , C 3 H 8 , C 4 H 6 , C 4 H 8 , C 4 H 10 , C 5 H 8 , C 5 H 10 , C 6 H 6 , C 6 H 10 , or C 6 H 12 , or two or more thereof. Alternatively, the process gas can further comprise an inert gas, such as a Noble gas (i.e., He, Ne, Ar, Kr, Xe, Rn). Alternatively yet, the process gas can further comprise O 2 , CO, NO, NO 2 , or N 2 O, or a combination of two or more thereof. 
     In  420 , plasma is formed in the plasma processing system from the process gas using, for example, any one of the systems described in  FIGS. 2 through 6 , and combinations thereof. 
     In  430 , the substrate comprising the residue, including but not limited to a photoresist layer or remnants of the photoresist layer, an ARC layer, or post-etch residue, is exposed to the plasma formed in  420 . After a first period of time, procedure  400  ends. The first period of time during which the substrate with the photoresist layer is exposed to the plasma can generally be dictated by the time required to ash the photoresist layer. In general, the period of time required to remove the residue is pre-determined. Alternately, the period of time can be further augmented by a second period of time, or an over-ash time period. As described above, the over-ash time can comprise a fraction of time, such as 1 to 100%, of the first period of time, and this over-ash period can comprise an extension of ashing beyond the detection of endpoint. 
     According to another embodiment, the plasma ashing process described in steps  410  through  430  may be preceded by a pre-treatment process. The pre-treatment process can be utilized to passivate various surfaces prior to performing the plasma ashing process to remove any remaining mask layer and/or other residue. 
       FIG. 8  presents a method of forming a feature in a dielectric layer on a substrate in a plasma processing system according to another embodiment of the present invention. The method is illustrated in a flowchart  500  beginning in  510  with forming the dielectric layer on the substrate. The dielectric layer can comprise an oxide layer, such as silicon dioxide (SiO 2 ), and it can be formed by a variety of processes including chemical vapor deposition (CVD). Alternately, the dielectric layer has a nominal dielectric constant value less than the dielectric constant of SiO 2 , which is approximately 4 (e.g., the dielectric constant for thermal silicon dioxide can range from about 3.8 to about 3.9). More specifically, the dielectric layer may have a dielectric constant of less than about 3.0, or a dielectric constant ranging from about 1.6 to about 2.7. 
     Alternatively, the dielectric layer can be characterized as a low dielectric constant (or low-k) dielectric film. The dielectric layer may include at least one of an organic, inorganic, and inorganic-organic hybrid material. Additionally, the dielectric layer may be porous or non-porous. For example, the dielectric layer may include an inorganic, silicate-based material, such as oxidized organosilane (or organo siloxane), deposited using CVD techniques. Examples of such films include Black Diamond™ CVD organosilicate glass (OSG) films commercially available from Applied Materials, Inc., or Coral™ CVD films commercially available from Novellus Systems. Additionally, porous dielectric films can include single-phase materials, such as a silicon oxide-based matrix having CH 3  bonds that are broken during a curing process to create small voids (or pores). Additionally, porous dielectric films can include dual-phase materials, such as a silicon oxide-based matrix having pores of organic material (e.g., porogen) that is evaporated during a curing process. Alternatively, the dielectric film may include an inorganic, silicate-based material, such as hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ), deposited using SOD techniques. Examples of such films include FOx HSQ commercially available from Dow Corning, XLK porous HSQ commercially available from Dow Corning, and JSR LKD-5109 commercially available from JSR Microelectronics. Still alternatively, the dielectric film can include an organic material deposited using SOD techniques. Examples of such films include SiLK-I, SiLK-J, SiLK-H, SILK-D, and porous SiLK semiconductor dielectric resins commercially available from Dow Chemical, and FLARE™, and Nano-glass commercially available from Honeywell. 
     In  520 , a patterned mask layer, such as a patterned layer of photoresist, is formed on the substrate overlying the dielectric layer. The mask layer may include a single layer or a plurality of layers. For example, the mask layer may include one or more soft mask layers and optionally one or more hard mask layers. For example, a layer of photoresist can be formed using conventional techniques, such as a photoresist spin coating system. The pattern can be formed within the photoresist film by using conventional techniques such as a stepping micro-lithography system, and a developing solvent. 
     In  530 , the mask layer pattern is transferred to the dielectric layer in order to form the feature in the dielectric layer. The pattern transfer is accomplished using a dry etching technique, wherein the etch process is performed in a plasma processing system. For instance, when etching oxide dielectric films such as silicon oxide, silicon dioxide, etc., or when etching inorganic low-k dielectric films such as oxidized organosilanes, the etch gas composition generally includes a fluorocarbon-based chemistry such as at least one of C 4 F 8 , C 5 F 8 , C 3 F 6 , C 4 F 6 , CF 4 , etc. or a fluorohydrocarbon-based chemistry or a combination thereof, and at least one of an inert gas, oxygen, and CO. Additionally, for example, when etching organic low-k dielectric films, the etch gas composition generally includes at least one of a nitrogen-containing gas, and a hydrogen-containing gas. The techniques for selectively etching a dielectric film, such as those described earlier, are well known to those skilled in the art of dielectric etch processes. 
     In  540 , the mask layer pattern, or remaining photoresist, or post-etch residue, etc., are removed. The removal of the mask layer is performed by exposing the substrate to a plasma formed of a process gas comprising CO 2 . The process gas can further comprise a passivation gas. For example, the passivation gas can include a hydrocarbon gas (C x H y ), wherein x, y represent integers greater than or equal to unity. The hydrocarbon gas (C x H y ) can include one or more of C 2 H 4 , CH 4 , C 2 H 2 , C 2 H 6 , C 3 H 4 , C 3 H 6 , C 3 H 8 , C 4 H 6 , C 4 H 8 , C 4 H 10 , C 5 H 8 , C 5 H 10 , C 6 H 6 , C 6 H 10 , or C 6 H 12 , or two or more thereof. Alternatively, the process gas can further comprise an inert gas, such as a Noble gas (i.e., He, Ne, Ar, Kr, Xe, Rn). Alternatively yet, the process gas can further comprise O 2 , CO, NO, NO 2 , or N 2 O, or a combination of two or more thereof. 
     Plasma is formed in the plasma processing system from the process gas using, for example, any one of the systems described in  FIGS. 2 through 6 , and the substrate comprising the mask layer is exposed to the plasma formed. A period of time during which the substrate with the photoresist is exposed to the plasma can generally be dictated by the time required to remove the photoresist. In general, the period of time required to remove the photoresist layer is pre-determined. However, the period of time can be further augmented by a second period of time, or an over-ash time period. As described above, the over-ash time can comprise a fraction of time, such as 1 to 100%, of the period of time, and this over-ash period can comprise an extension of ashing beyond the detection of endpoint. 
     In one embodiment, the transfer of the photoresist pattern to the dielectric layer, and the removal of the photoresist are performed in the same plasma processing system. In another embodiment, the transfer of the photoresist pattern to the dielectric layer, and the removal of the photoresist are performed in different plasma processing systems. 
     According to another embodiment, the plasma ashing process described in step  540  may be preceded by a pre-treatment process. The pre-treatment process can be utilized to passivate various surfaces, including surfaces of the dielectric layer, prior to performing the plasma ashing process to remove any remaining mask layer and/or other residue. The pre-treatment process may be utilized to passivate the side-walls of the feature formed in the dielectric layer in order to prevent a shift in the critical dimensions (CD) of the feature during the plasma ashing process. For example, the pre-treatment process can include exposing the mask layer and dielectric layer to plasma formed of a pre-treatment process gas comprising one or more hydrocarbon gases and, optionally, one or more inert gases, such as a noble gas or nitrogen. 
     According to yet another embodiment, the plasma ashing process described in step  540  may be adjusted in order to reduce the shift in CD of the feature formed in the dielectric layer during the plasma ashing process. For example, the flow rate of process gas to the region substantially above the center portion of the substrate may be varied relative to the flow rate of the process gas to the region substantially above the peripheral portion of the substrate. 
     Although only certain embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.