Abstract:
A method for controlling a plasma etch process while etching a layer stack having a first layer disposed above an end-point generating layer is disclosed. The method includes etching through the first layer and at least partially through the end-point generating layer while monitoring an absorption rate of a light beam traversing an interior portion of the plasma processing chamber, wherein the end-point generating layer is selected from a material that produces a detectable change in the absorption rate when etched. The end-point generating layer is characterized by at least one of a first characteristic and a second characteristic. The first characteristic is an insufficient thickness to function as an etch stop layer, and the second characteristic is an insufficient selectivity to etchants employed to etch through the first layer to function as the etch stop layer. The method additionally includes generating an end-point signal upon detecting the detectable change.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Application No. 60/421,422 entitled “Method and Apparatus for Absorption Endpoint Detection During Plasma Etch” by inventors Brian K. McMillin, Eric Hudson, and Jeff Marks (filed Oct. 24, 2002, Attorney Docket No. P1061P), which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates generally to methods for monitoring and controlling processes used in fabricating integrated circuit devices. More specifically, the invention relates to a method for detecting an endpoint during plasma etching of a film stack, such as a dielectric film stack.  
           [0003]    In the fabrication of integrated circuits, features may be etched into film stacks and filled with different materials to form the desired circuitry. To facilitate ease of understanding, the disclosure herein will focus on dual damascene integration as one exemplary technique involving the etching of a film stack. It should be understood, however, that the techniques disclosed herein are not limited only to dual damascene integration and may apply to the etching of any film stack wherein an endpoint is required.  
           [0004]    Generally speaking, dual damascene integration is used to form high-speed wiring interconnects in complex integrated circuit devices. In dual damascene integration, trenches and vias are formed in a low-permittivity (low-K) material, such as a fluorosilicate glass (FSG), an organosilicate glass (OSG), e.g., BLACK DIAMOND or CORAL, or a spin-on organic (SOO), e.g., SILK or FLARE, and filled with copper or another suitable conductor. The copper is used to reduce the resistance of the metal interconnect lines, and the low-κ material is used to reduce parasitic capacitance between the metal interconnects.  
           [0005]    [0005]FIG. 1A shows a dual damascene stack  100  formed on a copper line  102 . In general, a dual-damascene stack is made of a sequence of hard masks and interlayer dielectrics. In the discussions that follow, terms such as “above” and “below,” which may be employed herein to discuss the spatial relationship among the layers, may, but need not always, denote a direct contact between the layers involved. It should be noted that other additional layers above, below, or between the layers shown or discussed may be present. Further, not all of the shown or discussed layers need necessarily be present and some or all may be substituted by other different layers.  
           [0006]    For example, the dual damascene stack  100  includes a top hard mask  104 , a dielectric film stack  106 , and a diffusion barrier  108 . The dielectric film stack  106  includes low-κ interlayer dielectrics  110 ,  112  and an embedded hard mask  114 . The top hard mask  104  protects the interlayer dielectric  110  from chemicals used in photoresist stripping processes and may be omitted depending on the target application. The diffusion barrier  108  prevents undesirable diffusion of atoms between the copper  102  and the interlayer dielectric  112 . The embedded hard mask  114  and diffusion barrier  108  act as etch stop layers for the trench and via etches, respectively.  
           [0007]    There are two main approaches to dual-damascene integration: a trench-first approach and a via-first approach. In the trench-first approach, a via is patterned and etched in the dual-damascene stack  100  after a trench is formed in the stack  100 . In the via-first approach, a trench is patterned and etched in the dual-damascene stack  100  after a via is formed in the stack  100 . A less common approach to dual-damascene integration, called buried-via or self-aligned damascene process, involves patterning the embedded hard mask  114  prior to depositing the interlayer dielectric  110  and top hard mask  104  and etching the trench and via in the stack  100  in one step.  
           [0008]    FIGS.  1 B- 1 D illustrate an exemplary trench-first dual-damascene process. In FIG. 1B, a photoresist mask  116  having a trench pattern is applied on the top hard mask  104 . A trench  118  is formed in the stack  100  by etching the trench mask  116  through the top hard mask  104  and through the interlayer dielectric  110 , stopping on the embedded hard mask  114 . In FIG. 1C, the trench mask ( 116  in FIG. 1B) has been stripped off, and a photoresist mask  120  having a via pattern is applied on the top hard mask  104  and the exposed embedded hard mask  114 . A via  122  is formed in the stack  100  by etching the via mask  120  through the etch stop layer  114  and interlayer dielectric  112 , stopping on the diffusion barrier  108 .  
           [0009]    In FIG. 1D, the exposed diffusion barrier  108  has been opened up via an etch process, and the via mask ( 120  in FIG. 1C) has been stripped off. Copper  124  is deposited into the trench  118  and the via  122  and polished back to the surface of the trench  118 . The trench  118  and via  122  are typically lined with a material such as tantalum to prevent copper from diffusing into the interlayer dielectrics  110 ,  112 . In FIG. 1E, a blanket of diffusion barrier  125  is deposited on the stack  100  to cap the copper  124 .  
           [0010]    FIGS.  1 F- 1 I illustrate a via-first dual-damascene process. In FIG. 1F, a photoresist mask  126  having a via pattern is applied on the top hard mask  104 . A via  128  is formed in the stack  100  by etching the via mask  126  through the top hard mask  104 , the interlayer dielectric  110 , the embedded hard mask  114 , and the interlayer dielectric  112 , stopping on the diffusion barrier  108 . In FIG. 1G, the photoresist mask ( 126  in FIG. 1F) has been stripped off, and a photoresist mask  130  having a trench pattern is applied on the top hard mask  104 . A trench  132  is formed in the stack  100  by etching the trench mask  130  through the top hard mask  104  and the interlayer dielectric  110 , stopping on the embedded hard mask  114 .  
           [0011]    In FIG. 1H, the exposed bottom hard mask  114  has been opened up via an etch process, and the trench mask ( 130  in FIG. 1G) has been stripped off. Copper  134  is deposited into the via  128  and trench  132  and polished back to the surface of trench  132 . The via  128  and trench  132  are typically lined with a material such as tantalum to prevent copper from diffusing into the interlayer dielectrics  110 ,  112 . In FIG. 1I, a blanket of diffusion barrier  135  is deposited on the stack  100  to cap the copper  134 .  
           [0012]    In a trench etch, for example, the embedded hard mask  114  typically remains in the dielectric film stack  106   r . When used as an etch stop layer, the embedded hard mask  114  is required to have selectivity to etching, which typically means that the embedded hard mask  114  has a significantly slower etch rate compared to the interlayer dielectric  110 . Commonly, materials used as etch stop layers, such as SiN x  or SiC, tend to have a high permittivity (κ) value relative to the low-K interlayer dielectrics, which raises the overall κ value of the dielectric film stack  106 . An increase in the overall κ value of the dielectric film stack  106  results in an increase in parasitic capacitance and compromises the ability of the stack to mitigate electrical delays. Thus, it is desirable to eliminate the high-K embedded hard mask  114  or, at least, reduce its contribution to parasitic capacitance. However, control of etching is difficult when there is no effective etch stop layer.  
           [0013]    One method for controlling etching when there is no effective etch stop layer is timed-etch. However, timed-etch may have a very low yield because it does not account for incoming material variations, e.g., variations in film thicknesses from one stack to the next, differences in etch rates as a result of material composition, and differences in etch rates of substantially identical etch systems.  
           [0014]    Single or multiple discrete wavelength interferometry is an example of an optical diagnostic method that does not require an etch stop layer to detect an endpoint in an etching process. In single-wavelength interferometry, a light beam is directed on the surface of the wafer. The reflected signals from the wafer then combine constructively or destructively to produce a periodic interference fringe. When a predetermined number of fringes corresponding to the thickness of material to be removed has been counted, the etching process is stopped. In general, strong reflections from the interfaces (or underlying metal features) in a dual damascene structure make it difficult to use the interferometric approach for detecting an etching endpoint. Further, the interferometric approach has limited capability to account for incoming material variations because it measures relative changes in feature dimensions as opposed to absolute feature dimensions.  
           [0015]    From the foregoing, there is desired a method for detecting an endpoint during etching of a film stack without significantly increasing the overall κ value of the film stack.  
         SUMMARY OF THE INVENTION  
         [0016]    The invention relates, in one embodiment, to a method for controlling a plasma etch process in a plasma processing chamber. The method includes introducing a substrate having thereon a layer stack into the plasma processing chamber, the layer stack including a first layer disposed above an end-point generating layer. The method also includes etching through the first layer and at least partially through the end-point generating layer while monitoring an absorption rate of a light beam traversing an interior portion of the plasma processing chamber, wherein the end-point generating layer is selected from a material that produces a detectable change in the absorption rate when etched. The end-point generating layer is characterized by at least one of a first characteristic and a second characteristic. The first characteristic is an insufficient thickness to function as an etch stop layer for the etching, and the second characteristic is an insufficient selectivity to etchants employed to etch through the first layer to function as the etch stop layer. The method additionally includes generating an end-point signal upon detecting the detectable change.  
           [0017]    In another embodiment, the invention relates to a substrate configured to be etched in a plasma processing chamber. The substrate includes a first layer, and an end-point generating layer disposed below the first layer. The end-point generating layer is selected from a material that produces a detectable change in an absorption rate of a light beam traversing the plasma processing chamber when the end-point generating layer is etched. The end-point generating layer is characterized by an insufficient thickness/selectivity combination to function as an etch stop layer for a plasma etch that also etches through the first layer.  
           [0018]    In yet another embodiment, the invention relates to a method, in a plasma processing chamber, or etching through a low-κ layer of a layer stack, the low-K layer being disposed above an end-point generating layer. The method includes introducing a substrate having thereon the layer stack into the plasma processing chamber. The method further includes generating a light beam that traverses an interior portion of the plasma processing chamber. The method additionally includes etching through the layer stack, including the low-κ layer, while monitoring an absorption rate of the light beam by a direct absorption measurement technique, wherein the end-point generating layer is selected from a material that produces a detectable change in the absorption rate when etched. The end-point generating layer is characterized by an insufficient thickness/selectivity combination to function as an etch stop layer for the etching. The method further includes generating an end-point signal upon detecting the detectable change.  
           [0019]    These and other features and advantages of the invention will be discussed in more detail in the following detailed description of the invention and in conjunction with the following figures.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The invention is illustrated by way of example, and not by way of limitation, in the figures accompanying the drawings, and in which like reference numerals refer to similar elements, and in which:  
         [0021]    [0021]FIG. 1A shows a dual-damascene stack prior to forming a trench and a via.  
         [0022]    [0022]FIG. 1B shows a trench formed in the dual-damascene stack of FIG. 1A.  
         [0023]    [0023]FIG. 1C shows a via formed in the dual-damascene stack of FIG. 1B.  
         [0024]    [0024]FIG. 1D shows the trench and via of FIGS. 1B and 1C, respectively, filled with copper.  
         [0025]    [0025]FIG. 1E shows a blanket of diffusion barrier deposited on the stack of FIG. 1D to cap the copper.  
         [0026]    [0026]FIG. 1F shows a via formed in the dual-damascene stack of FIG. 1A.  
         [0027]    [0027]FIG. 1G shows a trench formed in the dual-damascene stack of FIG. 1F.  
         [0028]    [0028]FIG. 1H shows the via and trench of FIGS. 1F and 1G, respectively, filled with copper.  
         [0029]    [0029]FIG. 1I shows a blanket of diffusion barrier deposited on the stack of FIG. 1H to cap the copper.  
         [0030]    [0030]FIG. 2 shows a film stack including an endpoint-generating layer according to an embodiment of the invention.  
         [0031]    [0031]FIG. 3A is a schematic of an etch system equipped with a single-pass direct absorption measurement apparatus according to an embodiment of the invention.  
         [0032]    [0032]FIG. 3B is a schematic of an etch system equipped with a multi-pass direct on measurement apparatus according to an embodiment of the invention.  
         [0033]    [0033]FIG. 3C is a schematic of an etch system equipped with a cavity ring-down on measurement apparatus.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art, that the invention may be practiced without some or all of these specific details. In other instances, well-known process steps and/or features have not been described in detail in order to not unnecessarily obscure the invention. The features and advantages of the invention may be better understood with reference to the drawings and discussions that follow.  
         [0035]    In accordance with one aspect of the invention, there is provided with the film stack an end-point generating layer. Unlike the etch stop layer of the prior art, the end-point generating layer does not have to have the requisite selectivity or thickness to stop the etching. In fact, since the reduction of parasitic capacitance is an important consideration for the use of the low-K dielectric material, the end-point generating layer may be selected from materials having a lower κ value, the same κ value, or a slightly higher κ value without consideration for etch selectivity and thickness requirements to stop the etch.  
         [0036]    In one embodiment of the present invention, the end-point generating layer may have the same chemical composition but with slightly different stoichiometry than that of the layer being etched. The stoichiometry only needs to be different enough to trigger a signal by the detection equipment when the etch reaches the end-point generating layer. In another embodiment of the present invention, the end-point generating layer may have the same chemical composition but enriched with one or more isotopes. The isotope type and isotope dopant concentration are selected to be at least sufficient to trigger a signal by the detection equipment when the etch reaches the end-point generating layer. In another embodiment of the present invention, the end-point generating layer may have a different chemical composition from those of the layer being etched. The difference in the chemical composition of the layers is such that the detection equipment is capable of detecting such difference to generate an end-point signal when the etch reaches the end-point generating layer.  
         [0037]    Unlike the conventional etch stop layer, the endpoint-generating layer results in negligible and/or advantageously low perturbation in the overall κ value of the film stack or any other electrical and/or chemical properties of the film stack. In one embodiment, the detection of the etching of the end-point generating layer is accomplished using direct absorption measurement techniques. In various embodiments of the invention, the detection may be performed using a single-pass configuration or a multi-pass configuration. In another embodiment of the invention, the detection of the etching of the end-point generating layer is accomplished using frequency-modulated spectroscopy. In yet another embodiment, the detection of the etching of the end-point generating layer is accomplished using wavelength-modulated spectroscopy. Detection of the etching of the end-point generating layer may also be accomplished using electrical measurement techniques, optical emission spectroscopy (OES), or other absorption-based methods such as Fourier Transform Infrared Spectroscopy (FTIR), or cavity ring down techniques if such is capable of detecting the presence of the end-point generating layer materials or their byproducts in the plasma.  
         [0038]    The various embodiments of the present invention and their advantages may be better understood with reference to the exemplary drawings and discussions that follow. For illustration purposes, FIG. 2 shows a film stack  200  according to an embodiment of the invention. The film stack  200  includes film layers  202 ,  204 . As an example, the film layers  202 ,  204  could be made of a low-κ material, such as a fluorosilicate glass (FSG), an organosilicate glass (OSG), or a spin-on organic (SOO). Examples of OSGs include BLACK DIAMOND and CORAL. Examples of SOOs include SILK and FLARE. In accordance with one embodiment of the invention, an endpoint-generating layer  206  is embedded in the film stack  200 , e.g., between the film layers  202 ,  204 , using, for example, a multi-recipe thin-film deposition process. For dual-damascene integration, the film stack  200  could be stacked on a copper line.  
         [0039]    In one embodiment, the endpoint-generating layer  206  is a thin film having the same chemical composition as the film layers  202 ,  204  but a slightly different stoichiometry from the film layers  202 ,  204 . Advantageously, only a slight difference in stoichiometry is needed (the exact degree of difference depends on the sensitivity of the detection technique and detection equipment employed) to avoid substantially modifying the electrical and/or chemical characteristic of the film stack  200 . For example, the film layers  202 ,  204  and the endpoint-generating layer  206  could be made of an OSG having a SiO x C y H z  composition, and the fraction of one or more of C, Si, O, and H can be increased in the endpoint-generating layer  206  to make layer  206  have a slightly different stoichiometry from the film layers  202 ,  204 .  
         [0040]    During plasma etching, the gas phase species monitored for the purpose of generating an endpoint signal would then depend on the chemical element(s) having a different relative proportion in the endpoint-generating layer  206  and the etchant gases. Typically, the etchant gases include oxygen and fluorine-containing gases. As an example, if the chemical element having a different relative proportion in the endpoint-generating layer  206  is C and/or H, then a change in the CO or HF plasma species concentration can be detected during etching and used to generate the endpoint signal. It should be noted that CO and HF may be observed in the plasma prior to etching the endpoint-generating layer  206 , but a noticeable change in the concentration of the CO or HF plasma species would be observed when layer  202  is etched through or when endpoint-generating layer  206  is at least partially etched.  
         [0041]    Thus, a change in concentration of a plasma species of interest, i.e., an etch byproduct, should occur when layer  202  is etched through or endpoint-generating layer  206  is etched. When this change in concentration is detected, an endpoint signal can be generated. As explained above, this change could be achieved by making the stoichiometry of the endpoint-generating layer  206  slightly different from the stoichiometry of the film layers  202  and/or  204 .  
         [0042]    In another embodiment of the invention, the endpoint-generating layer  206  is a thin film having the same chemical elements as the film layers  202 ,  204  but enriched with an isotope such that isotopic proportions of the element are substantially modified from the natural or typical proportions for that element. Note that the natural or typical level of the isotope chosen for enrichment may be zero or non-zero in the end-point generating layer prior to enrichment. Preferably, the enriched isotope in the endpoint-generating layer  206  is compatible with the film layers, e.g., the isotope in the endpoint-generating layer  206  could be an isotope of a chemical element in the film layers  202 ,  204 . For example, if the film layers  202 ,  204  and the endpoint-generating layer  206  are made of an OSG having SiO x C y H z  composition, then one or more of the following isotopes could be enriched in the endpoint-generating layer  206 : carbon-13 ( 13 C), deuterium ( 2 H or D), oxygen-17 ( 170 ), oxygen-18 ( 18 O), silicon-29 ( 29 Si), and silicon-30 ( 30 Si).  
         [0043]    During plasma etching, the gas phase species monitored for the purpose of generating an endpoint signal would then depend on the enriched isotope(s) included in the endpoint-generating layer  206  and the etchant gases. For example, if  13 C and/or D is included in the endpoint-generating layer  206 , then  13 Co or DF would be monitored during etching of the endpoint-generating layer  206 , and an endpoint signal could be generated when a change in the concentration of  13 CO or DF is observed. It should be noted that CO and HF will be observed prior to etching the endpoint-generating layer  206 , but a significant concentration of  13 Co or DF would only be observed when the endpoint-generating layer  206  is etched.  
         [0044]    In an alternate embodiment, instead of tagging the endpoint-generating layer  206  with an isotope, the film layer  202  overlying the endpoint-generating layer  206  or the film layer  204  underlying the endpoint-generating layer  206  is tagged or enriched with the isotope. In another embodiment, one of the layers ( 202  or  204 ) may be tagged or enriched with the isotope and the endpoint-generating layer  206  may be omitted altogether. As in the previous embodiments, the concentration of the plasma species of interest is monitored to determine when to generate an endpoint signal.  
         [0045]    In another embodiment of the invention, the endpoint-generating layer  206  is a thin film having a different chemical composition from the film layers  202 ,  204 . In general, the chemical composition of the endpoint-generating layer  206  is selected such that the overall chemistry of the film stack  200  is not substantially modified. As an example, the film layers  202 ,  204  and endpoint-generating layer  206  could all be made of a silicate material, while selecting the silicate material in the endpoint-generating layer  206  to be different from the silicate material in the film layers  202 ,  204 .  
         [0046]    As an example, if the film layers  202 ,  204  are both made of an OSG or a FSG, the endpoint-generating layer  206  could be made of borophosphosilicate glass (BPSG) or silicon dioxide formed from chemical vapor deposition of tetraethyl orthosilicate (TEOS). In the case of BPSG, the endpoint of the etch process could then be detected by monitoring the plasma for a change in concentration of BF, BH, PF, PH, BO, PO, or other B- and P-containing species. In the case of TEOS, the endpoint of the etch process could be detected by monitoring the plasma for a change in concentration of CO. If the film layers  202 ,  204  are made of an OSG, the endpoint-generating layer  206  could be made of a FSG, and the plasma could be monitored for a change in concentration of HF.  
         [0047]    Table 1 below shows examples of endpoint-generating layers for different low-κ dielectric materials.  
                                                               TABLE 1                           Examples of endpoint-generating layers       for low-κ dielectric materials                ENDPOINT-GENERATING LAYER                                        G   H       LOW-K   A   B   C                 17 O or     29 Si or       DIELECTRIC   D     13  C   Modified   D   E   F     18 O     30 Si       MATERIAL   tagged   tagged   stoichiometry   BPSG   FSG   TEOS   tagged   tagged               OSG   X   X   X   X   X   X   X   X       SOO   X   X   X               X       FSG           X   X       X   X   X                  
 
         [0048]    Consistent with the principles of the invention, the endpoint-generating layers listed in Table 1 do not result in a significant change in the overall κ value of the film stack when embedded in the film stack. Other endpoint-generating layers besides those listed in Table 1 are also possible. A general guideline for selecting a suitable endpoint-generating layer will now be discussed.  
         [0049]    In general, the endpoint-generating layer  206  is selected to be compatible with the film layers  202 ,  204 , i.e., the intention is not to significantly modify the chemical and/or electrical characteristic of the film stack  200 . The κ value of the endpoint-generating layer  206  could be lower than, similar to, or slightly higher than the κ value of the thin-films  202 ,  204 . In general, the lower the κ value of the endpoint-generating layer  206 , the better.  
         [0050]    The contribution of the endpoint-generating layer  206  to the overall κ value of the film stack  200  is a function of the κ value and thickness of the endpoint-generating layer  206 . If the κ value of the endpoint-generating layer  206  is comparable to or lower than the κ value of the film layers  202 ,  204 , the endpoint-generating layer  206  can be made relatively thick to ensure reliable endpoint detection without significantly increasing the overall κ value of the stack. However, if the κ value of the endpoint-generating layer  206  is higher than the κ value of the film layers  202 ,  204 , the endpoint-generating layer  206  may have to be made relatively thin to limit its contribution to the overall κ value to a small value. Since the endpoint-generating layer  206  does not have to function as an etch stop layer (i.e., having the requisite selectivity or thickness to stop the etch), the endpoint-generating layer only has to be as thick as needed to trigger detection by the chosen detection technique/equipment.  
         [0051]    It is important to stress again that the endpoint-generating layer  206  is not the same as an etch stop layer. For example, the endpoint-generating layer  206  is not required to have selectivity to etching while a conventional etch stop layer is required to have selectivity to etching. This aspect flexibly allows the endpoint-generating layer  206  to be selected to have a κ value that is similar to or lower than the κ value of the film layers  202 ,  204 , which results in only a small or negligible perturbation in the overall κ value of the film stack  200 .  
         [0052]    Further, etching of a feature in the thin film  202  overlying the endpoint-generating layer  206  does not stop when the endpoint-generating layer  206  is exposed. Instead, the exposed endpoint-generating layer  206  may be completely removed before an endpoint signal is generated. This reduces the number of steps needed to form a feature. In the case of the conventional etch stop layer, the etching stops when the etch stop layer is exposed. The exposed etch stop layer may need to be removed in a subsequent step.  
         [0053]    The conventional etch stop layer is typically required to have sufficient thickness to compensate for over-etching. In the present invention, over-etching is not a concern. Actually, it is necessary to etch the exposed endpoint-generating layer  206  to create the detectable change in concentration of plasma species of interest, which is used to generate the endpoint signal. Because the endpoint-generating layer  206  is not required to compensate for over-etching, it can be made to be thinner than a conventional etch stop layer while still allowing generation of the endpoint signal.  
         [0054]    The concentration of a plasma species, or an etch byproduct, depends on the combination of production and loss rates. The rate of production of a plasma species of interest will change as etching proceeds from the thin film  202  to the endpoint-generating layer  206 . In a preferred embodiment, this change is determined by monitoring absorption of the plasma species of interest using one of several sensitive detection techniques, such as direct absorption, wavelength-modulation spectroscopy, frequency-modulation spectroscopy, FTIR, or cavity ring down methods. Although the sensitivity may be lower, optical emission spectroscopy may also be employed.  
         [0055]    In general, absorption spectroscopy involves passing a light beam through the etching plasma. The wavelength of the light beam is chosen to be near one of the resonant absorption wavelengths of the plasma species of interest. A change in intensity of the light beam as a result of absorption by the plasma species of interest indicates a change in the concentration of the plasma species of interest. When this change is observed, an endpoint signal can be generated.  
         [0056]    [0056]FIG. 3A shows an etching system  300  suited for in-situ monitoring of plasma etching of a film stack according to an embodiment of the invention. Note that etching system  312  is only exemplary; the invention can be practiced in any plasma etcher, irrespective of the technology employed to ignite, sustain and/or control the plasma. The exemplary etching system  300  includes a light source  302 , a reaction chamber  304 , and a light detector  306 . The reaction chamber  304  has diametrically-opposed windows  308 ,  310 , which are optically transparent at a selected wavelength or wavelength range. A wafer  312  is mounted inside the reaction chamber  304 , between electrodes  314 ,  316 . The electrode  314  is energized by a RF supply (not shown), while the electrode  316  is grounded. In other embodiments, the top electrode  316  may be powered by an RF power source at the same or different RF frequency. Although FIG. 3A illustrates a capacitively coupled plasma reactor, the technique could be applied to other reactor types such as inductively coupled or microwave systems. The wafer  312  includes a film stack, such as a dielectric film stack, with an endpoint-generating layer, such as described above.  
         [0057]    In operation, appropriate etchant gases (not shown) are supplied into the reaction chamber  304  and energized to form a plasma  320  above the wafer  312 . In order to monitor in-situ a plasma species of interest, the light source  302  is controlled to direct a light beam  322  into the reaction chamber  304  through the window  308 . Although not shown, an optical system, such as a system of collimating lenses or optical fiber, may be used to channel the light beam  322  from the light source  302  to the window  308 . The light beam  322  passes through the plasma  320  before exiting the window  310 . The wavelength of the light beam  322  is chosen to be near one of the absorption wavelengths of the plasma species of interest. As such, if the plasma species of interest is present in the reaction chamber  304 , the light beam  322  may be partially absorbed after passing through the plasma  320 . The transmitted beam, indicated at  324 , is detected by the light detector  306 .  
         [0058]    In one embodiment, the light source  302  is a temperature-controlled laser diode capable of operating within the infrared, visible, and/or UV spectral regions. Several species of interest in plasma etching have absorption transitions in the near infrared, such as HF at 1.33 μm, CO at 1.567 μm, HCl at 1.747 μm, and O 2  at 0.761 μm. Other species such as CF, CF 2 , and CF 3 , for example, have fundamental transitions at longer wavelengths, but can be accessed in the near infrared by probing vibrational overtone transitions.  
         [0059]    In one embodiment, the laser diode  302  generates a wavelength- or frequency-modulated laser beam. Frequency-modulated spectroscopy uses modulation frequencies comparable to or greater than the absorption linewidths, whereas wavelength-modulated spectroscopy uses modulation frequencies much smaller than the absorption linewidths. The laser drive current  326  may be generated by a low-noise current driver  328  acting in response to a superposition of an input sawtooth waveform  330  (at, e.g., 100 Hz) generated by a signal generator  332  and an input sinusoidal waveform  334  (at, e.g., 50 KHz) generated by a signal generator  336 . The waveforms  330 ,  334  are summed by a circuit  338  before being input to the current driver  328 .  
         [0060]    The DC magnitude of the laser drive current  326  and the laser temperature, which is typically near room temperature, e.g., in a range from 0-50° C., determine the initial laser wavelength for the scan. The sawtooth component scans the laser wavelength through the absorption transition. The sinusoidal modulation facilitates use of phase-sensitive detection of the beam absorption. The laser wavelength is chosen to be near the absorption transition wavelength of the plasma species of interest and, typically, the sawtooth waveform  330  is chosen so that the laser wavelength scan width is comparable to the linewidth of the spectral feature being monitored.  
         [0061]    In one embodiment, the light detector  306  is a high-speed photodiode, and the laser frequency is locked to an individual rotational component of a vibration transition of the plasma species of interest to allow for sensitive detection even in the presence of high background electrical noise. In one embodiment, the locking scheme includes inputting a photodiode current  340  to a lock-in amplifier  342  along with a sinusoidal waveform  342  from the signal generator  336 . The lock-in amplifier  342  usually provides the capability to apply the appropriate gain, filtering, and phase adjustment for phase-sensitive detection of the photodiode current  340  at, for example, the modulation frequency, f, or at a harmonic, nf. The output of the lock-in amplifier  342  is then digitized by an analog-to-digital converter  344  and analyzed by a computer  346 .  
         [0062]    In operation, the relative change in photodiode current  340  is used to determine a change in absorption and, thus, concentration of the plasma species of interest during the etching process. For example, using a laser operating at a nominal wavelength of 1.33 μm, a change in the relative concentration of HF in the reaction chamber  304  can be detected by the relative change in photodiode current  340  detected by the photodiode  306 . The change in concentration of HF or other suitable etch byproduct, determined based on the endpoint-generating layer and the etchant gases, can be used as an indicator of the endpoint of the etch.  
         [0063]    The absorption detection technique described above involves passing the light beam  322  through the plasma  320  once, i.e., a single-pass configuration. For enhanced sensitivity, the light beam  322  may be passed through the plasma  320  multiple times, i.e., a multi-pass (or white-cell) configuration. Many of these detection techniques are also discussed in publications such as “Sensitive plasma etching endpoint detection using tunable diode laser absorption spectroscopy by H. S. Sun, V. Patel, B. Singh, C. K. Ng, and E. A. Whittaker (Applied Physics Letters, Vol. 64, Issue 21, pp. 2279-2781) and “Near-infrared diode lasers monitor molecular species” by David E. Cooper and Ramon U. Martinelli (Laser Focus World, November 1992), all of which are incorporated by reference.  
         [0064]    [0064]FIG. 3B shows an etching system  347  wherein two multi-pass mirrors  348 ,  350  are mounted outside the reaction chamber  304  in a diametrically-opposed fashion. The mirrors  348 ,  350  are mounted outside the reaction chamber  304  to avoid contamination of the wafer  312  during the etching process. The laser diode  302  directs a laser beam  351  to the mirror  348 . The laser beam  351  passes through the mirror  348  and bounces back and forth between the mirrors  348 ,  350 . The laser beam  351  passes through the plasma  320  as it bounces back and forth between the mirrors  348 ,  350 . A portion of the laser beam  351  exits the mirror  350  with each roundtrip. The transmitted laser beam, indicated at  353 , is detected by the photodiode  306  after the laser beam  351  makes a predetermined number of passes through the plasma  320 .  
         [0065]    [0065]FIG. 3C illustrates another etching system  355  wherein the reaction chamber  304  is disposed in an optical cavity  352 , which is formed by diametrically-opposed, highly-reflective concave mirrors  354 ,  356 . In operation, the light source  302  injects a laser beam  358  into the optical cavity  352  at a wavelength near the absorption transition wavelength of the plasma species of interest. Typically, the laser beam  358  is injected into the optical cavity  352  at a selected repetition rate to continuously monitor the evolution of a plasma species interest until the endpoint of the etch is detected. The laser beam  358  bounces back and forth between the mirrors  354 ,  356  many times until its intensity decays. Alternatively, chamber window  308  and/or chamber window  310  may include a reflective coating on the internal surfaces to serve the purpose of illustrated mirror  354  and/or mirror  356 . The laser beam  358  passes through the reaction chamber  304  and plasma  320  during each roundtrip between the mirrors  354 ,  356 . A portion of the laser beam  358  in the optical cavity  352  is transmitted through the mirror  356  with each roundtrip of the laser beam  358  and is detected by the light detector  306 . The transmitted beam, indicated at  360 , is displayed on an oscilloscope  362 .  
         [0066]    The time required for the intensity of the transmitted beam  360  to decay to 1/e of the intensity of the injected beam  358  is known as the “ring-down time” of the optical cavity  352 . The cavity ring-down time is a function of the length of the optical cavity  352 , the roundtrip transit time of the laser beam  358 , the magnitude of the intrinsic cavity losses, and the absorption of the plasma species of interest. By measuring the cavity ring-down time, the absorption of the plasma species of interest can be determined. The accuracy with which the cavity ring-down time is measured is sensitive to the intrinsic cavity losses. Therefore, it is important that the optical cavity  352  forms a stable resonator, e.g., by optically-matching the various embedded optical cavities.  
         [0067]    In yet another embodiment, the frequency modulated spectroscopy technique may be used to generate an endpoint signal from a “conventional” etch stop layer when performing a trench etch, and more particularly a dielectric trench etch in a plasma etcher. The use of frequency modulated spectroscopy on a conventional etch stop layer improves detection sensitivity in many cases, ensuring that the etching stops when intended with higher throughput.  
         [0068]    As mentioned, the rate of absorption of HF or CO may be measured and employed to generate an endpoint signal when etching through an overlying layer. Note that in some cases, it is not necessary to etch into the underlying layer (but such underlying layer may be etched if desired) to obtain an endpoint signal since the surface area of the overlying layer that is exposed to the etchant would be reduced when the overlying layer is etched through, thereby changing the composition of the etch byproducts generated. This change in the etch byproduct composition may then be employed to generate an endpoint signal. In this case, the underlying layer may represent any layer that is different from the overlying layer as long as the presence of the underlying layer can cause a detectable change in the etch byproduct when the etchant employed to etch the overlying layer etches through the overlying layer. Thus the provision of a separate end-point generating layer, although possible, is not absolutely necessary in these situations.  
         [0069]    For example, certain dual damascene layer stacks may employ a SiO 2  overlying layer disposed above a TEOS or BPSG layer. The trench would be etched in the SiO 2  layer while a via may be formed in the TEOS or BPSG layer, for example. In this case, the concentration of HF or CO would change when the overlying SiO 2  layer is etched through, thereby triggering the generation of an endpoint signal. Of course the change in the absorption rate due to the changing concentration of HF or CO when the etch proceeds into the underlying TEOS or BPSG layer may also be employed to generate the endpoint signal, if desired.  
         [0070]    As another advantageous application example, the absorption rate of HF (or CO) may be monitored to generate an endpoint signal in a low-K dual-damascene application, such as the case when a low-K layer is disposed above another layer. As an example, some dual damascene applications may involve an OSG layer (low-K) is disposed above a FSG layer (slightly higher K than the OSG layer). In this case, a trench may be formed in the OSG layer, while the via would be disposed in the FSG layer. This arrangement is feasible since the line-to-line capacitance at a given level may be more significant than the level-to-level capacitance (due to the smaller line-to-line spacing). By monitoring either the absorption rate based on the concentration of CO or HF, it is possible to detect when one layer (e.g., OSG) is etched through, or when the etch proceeds into an underlying layer (e.g., FSG). OSG is provided as one example of a low-K material although another suitable low-κ material may well be employed. Likewise, FSG is provided as one example of a layer within which the via of the dual-damascene is formed, although another suitable layer may well be employed.  
         [0071]    As can be appreciated from the foregoing, the invention provides numerous advantages. One key advantage stems from the use of an endpoint-generating layer, which does not have to have the selectivity and/or thickness requirement of an etch stop layer, in generating an endpoint signal. The use of such an endpoint-generating layer is advantageous because such endpoint-generating layer can be designed such that there is little perturbation in the overall κ value and/or other electrical and/or chemical characteristic of the film stack. This is especially useful in dual damascene integration, where an increase in the overall κ of a dielectric film stack can compromise the stack&#39;s ability to mitigate electrical delays.  
         [0072]    Another key advantage is flexibility in selecting the thickness of the endpoint-generating layer since the endpoint-generating layer is not required to compensate for over-etch. This is particularly advantageous in modem, high density IC applications where features are increasingly scaled with narrower design rules, and thinner film stacks facilitate such high density applications. As long as the chosen detection technique and/or equipment can detect the change during etching, the thickness of the end-point-generating layer and/or the difference between the etched layer and the endpoint-generating layer may be as minimal as desired.  
         [0073]    Furthermore, it may cost less to provide the endpoint-generating layer. A lower cost material than the etch stop layer material may be employed and/or the process to dope or deposit the endpoint-generating layer may be less costly. In some cases, fewer steps are required to create the endpoint-generating layer compared to the number of steps required to deposit the etch stop layer. Thus, the resultant circuit may be created in a shorter amount of time and/or with less cost.  
         [0074]    While the invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the invention.