Abstract:
A substrate etching apparatus comprises a chamber having a wall with a window, substrate support pedestal, energy source, and monitoring assembly with signal sensor capable of detecting reflected radiation from the substrate from directly above the substrate after the radiation propagates through the window in the wall. An etching method comprises the steps of: providing a substrate in a chamber, etching a channel or trench in the substrate by coupling energy through the wall of the chamber to energize an etch gas in the chamber, detecting radiation reflected from the substrate from directly above the substrate after the radiation propagates through the wall and evaluating the detected radiation to monitor the depth of etching of the channel or trench being etched on the substrate.

Description:
CROSS-REFERENCE 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 09/595,778, filed on Jun. 16, 2006, which is a divisional of U.S. patent application Ser. No. 08/944,240, filed on Oct. 6, 1997, which issued as U.S. Pat. No. 6,129,807 on Oct. 10, 2000, both of which are incorporated by reference herein in their entireties. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the present invention relates to monitoring processing of a substrate in a processing chamber. 
         [0003]    Semiconductor processing systems that perform “dry” etching of semiconductor wafers via plasmic gases, also known as reactive ion etching (RIE) require constant monitoring. While it is possible to predefine the etch parameters and allow the systems to perform the etch process unmonitored, conditions within the systems can change over time. Minute changes in the composition or pressure of an etch gas or process chamber or wafer temperature creates undesirable etch results. 
         [0004]    For example, DRAM memory circuits are fabricated from semiconductor wafers using deep trench technology. A single DRAM memory cell consists of a capacitive storage cell and a switching element (i.e., a MOSFET transistor). Information (in the form of electrical charge) stored in the cell is passed on to other circuitry when the switching element is activated. Essentially very deep (on the order of 3-20 mm) channels or trenches must be formed in a semiconducting substrate in order to create the capacitive storage cells. Otherwise, the information is not sustained (i.e., the electrical charge “leaks out” of the storage cell). 
         [0005]    Such trench etch circuits are formed by etching away different layers of insulating material deposited upon the substrate and the substrate itself in various steps. For example, first a photoresist mask is placed over an insulating layer or film (silicon dioxide or other similar material). The mask contains a desired circuit pattern to be etched into the insulating layer. It is important that etching of the insulating layer stop at the point where the substrate (silicon or other similar composition) is first revealed at the bottom of the trench. In a next step, the remaining portion of the photoresist mask is removed via an ashing operation so as to not remove any of the remaining insulating film or improperly etch the substrate. In a next step, a more involved chemical process etches a trench into the substrate material while continuously redepositing the insulating layer material so as to not attack the original insulating layer defining the circuit pattern. It can easily be seen that if the etch process during any one step exceeds the predetermined endpoint, the substrate, insulating layer and/or resultant circuit pattern may be damaged. As such, these systems rely upon some type of in situ measurement to determine the progressive depth of the etch process. In situ measurement provides greater control of the etch process and improves uniformity over a batch of processed wafers. 
         [0006]    There has been some success in the art of developing in situ etch depth measuring systems that utilize optical emission spectroscopy to monitor light emissions from the plasma as the etch process progresses. One such system is disclosed in U.S. Pat. No. 5,308,414 to O&#39;Neill et al. Such a system monitors the optical emission intensity of the plasma in a narrow band as well as a wide band and generates signals indicative of the spectral intensity of the plasma. When the signals diverge, a termination signal is generated thereby terminating the etch process. Other techniques include the use of laser interferometry, beamsplitters and diffraction gratings to measure the phase shift of a laser beam reflected from two closely spaced surfaces. For example, the phase shift between a first beam reflected off the mask pattern and the beam reflected off an etched portion of the wafer is measured and compared to a predetermined phase shift that corresponds to the desired etch depth. Unfortunately such monitoring and measuring systems are plagued by inadequate signal to noise ratios. Additionally, the minimum etch depth is limited by the wavelength of the light source used in the monitor. Another technique for measuring etch depth is ellipsometry, which measures the change in polarization of light upon reflection of the light from a surface. Unfortunately, the error in etch depth detection in systems that use randomly polarized laser beams instead of linearly polarized beams is too great to be useful. 
         [0007]    In situ etch depth monitoring is of particular interest in systems where plasma excitation coils are used. Such a system is the Decoupled Plasma Source (DPS) system manufactured by Applied Materials, Inc. of Santa Clara, Calif. For example, RF power applied to a coil configuration atop a process chamber assists in creating the plasma that performs the etch process. However, the RF power may inductively couple into the neighboring monitoring equipment thereby corrupting the monitoring signals. As such, in situ monitoring of etch depth in a high power RF environment is inadequate and prone to severe inaccuracy. 
         [0008]    Therefore, a need exists in the art for an apparatus for performing direct, in situ measurement of etch depth in a high power RF environment as well as monitoring other processed performed by a semiconductor wafer processing system. 
       SUMMARY 
       [0009]    A method of etching a substrate in a chamber having a wall and detecting an endpoint of the etching process comprises the steps of: providing a substrate in the chamber, etching a channel or trench in the substrate by coupling energy through the wall of the chamber to energize an etch gas in the chamber, detecting radiation reflected from the substrate from directly above the substrate after the radiation propagates through the wall and evaluating the detected radiation to monitor the depth of etching of the channel or trench being etched on the substrate. 
         [0010]    An etching apparatus for etching a substrate is provided, the apparatus comprising an etch chamber, substrate support pedestal upon which a substrate can be retained, energy source to couple energy to an etch gas in the chamber to form a plasma to etch a channel or trench in the substrate and process monitoring assembly to monitor a depth of the channel or trench being etched in the etch chamber. The process monitoring assembly comprises a signal sensor capable of detecting radiation reflected from the substrate from directly above the substrate after the radiation propagates through the window in the wall. 
     
    
     
       DRAWINGS 
         [0011]    These features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, which illustrate examples of the invention. However, it is to be understood that each of the features can be used in the invention in general, not merely in the context of the particular drawings, and the invention includes any combination of these features, where: 
           [0012]      FIG. 1  depicts a schematic representation of a high power RF etch chamber; 
           [0013]      FIG. 2  depicts a partially sectional, perspective view of the upper portion the etch chamber; 
           [0014]      FIG. 3  depicts a schematic representation of a high power RF etch chamber containing a second embodiment of an apparatus; 
           [0015]      FIG. 4  depicts a partially sectional, perspective view of the upper portion of the etch chamber containing a second embodiment of the apparatus; 
           [0016]      FIG. 5  depicts a schematic representation of an etch chamber containing a third embodiment of the apparatus; and 
           [0017]      FIG. 6  depicts a schematic representation of an etch chamber containing a fourth embodiment of the apparatus. 
       
    
    
     DESCRIPTION 
       [0018]    An apparatus is provided for performing direct, in situ monitoring of processes such as etch depth of and thin film deposition upon a semiconductor wafer within a semiconductor wafer processing system. To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
         [0019]    The apparatus provides for measurement of a variety of emissia or reflected light dependent upon chamber conditions and monitoring apparatus preferences and parameters. Specifically, the apparatus is used for monitoring the depth of various types of etch processes from within a dome temperature control enclosure of a Metal Etch Decoupled Plasma Source (DPS) chamber manufactured by Applied Materials of Santa Clara, Calif. A dome temperature control enclosure and apparatus of the Metal Etch DPS chamber is disclosed in U.S. patent application Ser. No. 08/767,071, filed Dec. 16, 1996, and is herein incorporated by reference. The processes that can be monitored include but are not limited to gate etch, recess etch, deep trench and shallow trench isolation for the production of DRAM memory and logic circuits. 
         [0020]      FIG. 1  depicts a schematic representation of the apparatus in its operating environment, e.g., measuring etch depth as a semiconductor wafer is etched. Specifically, a process chamber  100  is defined by sidewalls  102 , a bottom  104  and a dome  106 . The chamber  100  houses a substrate support pedestal  108  upon which a substrate (i.e., a semiconductor wafer)  110  is retained. An etch process is performed on the wafer  110  to create a desired integrated circuit pattern or the like. During the etch process, temperature control of the dome  106  is critical to proper etching of the wafer. As such, an additional enclosure  114  is defined by sidewalls  113  and cover  112  above the dome  106 . Within this enclosure  114  is a temperature control apparatus  116 . A temperature control apparatus  116  maintains the temperature of the dome  106  within a preferred, optimum operating range. 
         [0021]    Additionally, the enclosure  114  also houses a device for monitoring the processing, e.g., depth of the etch process that occurs at the wafer surface. Specifically, a collimating assembly  126  is disposed above the dome  106 . A signal source  118  and signal sensor  120  are connected to the collimating assembly  126  via a transmission cable  128 . While a fiber optic cable is a preferred device for connecting the signal source  118  and signal sensor  120  to the collimating assembly  126 , any suitable transmission cable may be used. The combination of the collimating assembly  126 , the signal source  118 , signal sensor  120  and transmission cable  128  comprise a monitoring assembly  121 . 
         [0022]    In one version, a second, bifurcated end of the fiber optic cable has two branches. A first branch  127  of the bifurcated end of the fiber optic cable is attached to the signal source  118  and a second branch  129  is attached to the signal detector  120 . Both the signal source  118  and signal detector  120  are outside the enclosure  114 . A first, single end of a fiber optic cable extends through an opening  130  in the sidewall  113  of the enclosure  114  and is attached to the collimating assembly  126 . The transmission cable  128  and collimating assembly  126  may be provided with shielding elements  134  and  136  respectively to avoid RF power (explained in greater detail below) and excessive temperature from coupling into or effecting these devices. Alternately, the devices are manufactured from non-conductive materials such as high temperature plastics, ceramics and the like or are a combination of shielded and non-conductive components. 
         [0023]    The top of the dome defines an apex  123 . An opening  122  is bored into the dome  106  proximate the apex  123 . To maintain the integrity of the chamber conditions during wafer processing, a window  124  is placed in the opening  122 . Preferably, the window  124  is a slab of transparent material having a low refractive index so as to prevent excessive refraction of an optical beam. Materials such as quartz and sapphire can be used to create the window. Fused silica is also a viable window material because it has a higher transmissibility of ultraviolet light than ordinary glass. Ideally, the window  124  and dome  106  are machined to high tolerances so as to create a flush mounting surface. Specifically, the opening  122  in the dome  106  has a flange thereby providing a supporting lip  138  upon which the window rests. 
         [0024]    The window may be permanently adhered to the dome or removable therefrom. If the window is permanently adhered to the dome, an adhesive is used along the supporting lip  138  of the dome  106  to affix the window  124  and maintain chamber conditions. Alternately, the window  124  is fused or welded to the opening  122 . If the window is removable, both the window and the opening are specially prepared. Specifically, the supporting lip  138  and the window  124  are polished. The two polished surfaces are sealed with an O-ring (not shown) placed between the supporting lip  138  and the window  124 . As such, an air-tight seal is formed when a vacuum produced in the chamber  100  draws the window  124  down onto the supporting lip  138 . In one version, the window  124  is permanently affixed to the dome  106 . A permanent window is affixed to the opening during manufacture of the dome and is constructed of a material that is specific to the type of monitoring apparatus used in the enclosure. For example, a laser interferometer is used in combination with a window comprised of sapphire. 
         [0025]    As described above, a wide angle, line-of-sight measurement can be taken as the wafer is being processed. In another embodiment, the signal source  118  is an optical source capable of emitting an optical beam of sufficient wavelength, frequency and amplitude to propagate through the chamber processing environment without excessive levels of signal degradation or interference. Preferably, a low pressure, mercury-based plasma lamp operating in the 185-700 nm range is used as the signal source. Alternately, cadmium, zinc or other plasma-based or laser-based lamps may be used for the signal source in place of the mercury-based plasma lamp. An optical beam from the signal source  118  travels through the first branch  127  of the bifurcated end of the fiber optic cable  128  to the collimating assembly  126 , through the window  124  and onto the wafer  110 . A relatively large (i.e., approximately 1 square inch diameter) area of the wafer encompassing at least one entire die pattern being etched is illuminated by the optical beam. As such, a larger area is available for etch depth monitoring which provides greater accuracy in determining the overall etch rate of the wafer. 
         [0026]    The signal sensor  120  is an optical sensor capable of receiving reflected beams from the wafer  110  that have propagated through the chamber processing environment. Preferably, the signal sensor is a narrow band (approximately 2 nm) monochromator with a silicon photodiode or photomultiplier. In an alternate embodiment, the signal sensor is a photomultiplier with a narrow band (approximately 2 nm) optical filter placed in front of the photomultiplier. The optical filter&#39;s multiple layers of dielectric film function as a band pass filter. That is, desired wavelengths of reflected beams from the wafer pass through the optical filter while all over wavelengths are screened out. For example, light from the plasma within the chamber does not enter the photomultiplier. This type of filtering greatly enhances the signal-to-noise ratio of the reflected beams. Specifically, a reflected beam from the wafer  110  propagates through the process chamber  100 , window  124 , collimating assembly  126 , into the fiber optic cable  128  and exiting at the second branch  129  of the bifurcated end of the fiber optic cable  128  and into the signal sensor  120 . The signal sensor  120  processes the reflected signal into an etch rate signal that may be passed on to a computer (not shown) for additional processing, display device (not shown) to depict progress of wafer processing or the like. Alternately, the signal detector  120  may be a CCD camera to form part of an image relay system. 
         [0027]    The above described monitoring assembly  121  need not be designed from separate components interconnected by a transmission cable.  FIG. 6  depicts a simplified schematic representation of an alternate embodiment whereby the signal source  118  and signal detector  120  comprise a single monitoring unit  600 . Specifically, the signal source  118  and signal detector  120  are oriented at an angle of 90° from one another with a beamsplitter  602  and additional lens assembly  604  acting as a signal relay interface. The monitoring unit  600  may then be connected to the collimating assembly  126  via a non-bifurcated fiber optic cable  606  or other similar transmission cable. This type of configuration is especially useful when using short wavelength light as the signal source. 
         [0028]      FIG. 2  depicts a detailed partial sectional, perspective view of the enclosure  114 . Specifically, the enclosure  114  is bounded by a cylindrical sidewall  113  extending vertically from a circumferential edge of the dome  106  to the cover  112 . A portion of the temperature control apparatus  116  extends from an inner wall  202  of the enclosure  114  towards the center terminating at an annular lip  204 . Other portions of the temperature control device have been omitted from the figure for clarity. A support bracket  132  is secured to the dome  106  and circumscribes the opening  122 . The collimating assembly is attached to the support bracket  132  to support the collimating assembly  126  above the window  124 . Preferably the support bracket is fabricated from a high temperature plastic such as Ultem® (a registered trademark of General Electric). 
         [0029]    Aside from forming the lower extremity of the enclosure  114 , the dome  106  also defines a surface  212  that supports an RF antenna  210 . Specifically, a single length of a conductor (i.e., a copper coil) is positioned at the circumference of the dome  106  and coiled radially inward. The antenna coil covers approximately ⅔ of the support surface  212 . The antenna  210  is coupled to a high power RF power source (not shown) for the purpose of ionizing a process gas into a plasma in the process chamber  100 . The antenna  210  and the RF source form a decoupled plasma source. Preferably, the dome  106  is opaque quartz or a ceramic such as alumina. Such materials are substantially transparent to infrared wavelengths that are produced by lamps within the temperature control unit. The heat produced by these emissions are used to heat the chamber environment. As such, the dome  106  is permeable to the magnetic fields from the antenna  210  which control and enhance plasma characteristics. Proximate the apex  123  of the dome  104 , the opening  122  is formed. As such, the beams from and to the collimating assembly  126  pass through the opening  122  and into and out of the process chamber  100 . 
         [0030]    As discussed earlier, one apparatus embodiment has a permanently affixed window. In an alternate embodiment of the apparatus, the window is removable from the dome. A removable window adds flexibility to the apparatus in that different types of signal sources and sensors can be used in the same chamber. For example, a chamber using a low pressure, mercury-based plasma lamp and a sapphire window can be retooled to accept a laser interferometer and a quartz window. The material chosen for the window is based upon the wavelength of the beams used in the monitoring assembly. Although mercury lamps, laser interferometers and X-rays are discussed, any type of optical beam equipment can be used. Similarly, any type of material besides sapphire and quartz can be used for the window to optimize transmission of the beams, refraction index and general operation of the device. For example, quartz is more etch resistant than sapphire, but sapphire has a lower cost and different transmission bandwidth than sapphire. Although methods of permanently and removably affixing the window to the dome are discussed any means for affixing the window to the dome can be used to optimize the ability to retool the chamber or obtain adequate measurements from the monitoring assembly. 
         [0031]      FIGS. 3 and 4  depict a further embodiment of the inventive apparatus. Specifically,  FIG. 3  depicts a schematic representation of the etch chamber  100  with enclosure  114  and temperature control apparatus  116  similar to that depicted in  FIG. 1 . However, in this embodiment, the signal source  118  and signal detector  120  are mounted inside the enclosure  114 . The signal source  118  and signal detector  120  are provided with shielding  136  from RF sources and excessive temperatures and are disposed directly above the collimating assembly  126 . Specifically, the signal source  118  and signal detector  120  are mounted to the collimating assembly  126 . As indicated previously, the collimating assembly  126  can also be shielded and held secure to temperature control apparatus  116  via support bracket  132 . This all-internal configuration is also shown in a partially sectional perspective view in  FIG. 4 . As can be seen from either figure, this configuration provides an elegant and highly simplified solution to in-situ measurement. Specifically, no external components are used in this embodiment thus eliminating the need for a fiber optic (or similar signal transmission) cable and for the additional hole ( 130  of  FIGS. 1 and 2 ) in the enclosure. 
         [0032]    The signal source  118  and signal detector  120  need not be disposed in a side-by-side arrangement. For example,  FIG. 5  depicts an alternate embodiment of the monitoring assembly  500  whereby the signal source  118  and signal detector  120  are oriented at an angle of 90° from each other with a modified collimating assembly functioning as a relay interface between the signal source  118  and signal detector  120 . The modified collimating assembly contains additional optical devices (i.e., folding mirrors and/or lens  502 ) to properly direct the signal source and reflected beams. From this discussion one skilled in the art can design such an internal monitoring assembly in a variety of configurations. In the spirit and scope of this specification the monitoring assembly is in no way limited to the described configurations. The signal source, signal detector, collimating assembly and attendant hardware can be arranged in any configuration necessary to create a monitoring assembly that is totally internal to the enclosure or similar chamber top surface. 
         [0033]    With the configuration as described, an optical measurement apparatus is created that is capable of in-situ monitoring of the etching process. Specifically, an optical beam from the source  118  propagates through the window  124  and chamber  100  to the wafer surface. An incident beam reflects from the wafer surface, propagates back through the window  124 , collimating assembly  126 , transmission cable  128  and is detected by the sensor  120 . Since the monitoring assembly  121  is shielded and/or positioned away from the antenna coils, interference or RF power coupling is minimized. All or part of the monitoring assembly may also be in close proximity to heat lamps which are part of the temperature control apparatus. Reducing thermal expansion of monitoring components is important so as to reduce the likelihood of misalignment of the apparatus. Such misalignment can lead to erroneous signal detection. The shielding element  130  can be fashioned as sleeve surrounding the transmission cable  128  fabricated from polyetheretherketone. The shielding element  136  can be a metallic plate with a painted or otherwise applied dielectric coating. Additionally, the improved apparatus requires no major retooling of existing chamber components. The design increases flexibility by allowing use of different types of process monitoring equipment in the same chamber. 
         [0034]    The above described apparatus is not limited to use inside an enclosure above a wafer process chamber. Nor does the window or opening in which the window is fitted need to be part of or affixed to a dome shaped top surface. The top surface may be flat, concave or any configuration suitable for sealing the process chamber. The window  124  and opening  122  need not be at the apex of the dome or similar top surface as they can be off center. Additionally, there need not be only one window and corresponding opening. There may be a plurality of openings in the top surface each covered by a separate window or all covered by a single plate disposed above the top surface. In such a configuration having a plurality of openings and windows, there can be a single source providing illumination at all openings or a plurality of sources providing illumination to a group of openings or to each opening individually. Accordingly, there can be a single detector receiving reflected beams passing through all of the openings. Alternately, there can be a plurality of detectors receiving reflected signals from a group of openings or from each opening individually. 
         [0035]    Thus, the disadvantages associated with the prior art are overcome by the present apparatus for performing direct, in situ monitoring of a process in a semiconductor wafer processing system. In one example, the apparatus provides a process chamber having a dome circumscribed by an antenna, the dome having an opening, an enclosure disposed above the chamber, a process monitoring assembly disposed proximate said dome and a window covering the opening. Further, the process monitoring assembly consists of a signal source, a signal detector, a collimating assembly and a transmission cable having a first end and a second end whereby the first end is connected to the collimating assembly and the second end is connected to the signal source and the signal detector. A portion of the apparatus supports the process monitoring assembly to establish a line-of-sight from the monitoring assembly, through the window to a substrate (i.e., a semiconductor wafer). The window can be permanently affixed to the opening or removable. The monitoring apparatus can be located totally within the enclosure or a portion of it can be outside of the enclosure. 
         [0036]    A method of fabricating the exemplary apparatus comprises boring an opening proximate an apex in the dome, positioning the process monitoring assembly in proximity to the dome so as to allow a line-of-sight from the process monitoring assembly to a wafer, and covering the opening with a window. The window is permanently affixed or removable dependent upon the type of process monitoring apparatus being used in the system. 
         [0037]    With the method and apparatus as disclosed, process measurement and monitoring is conducted without encountering interference from high power energy sources proximate the chamber. Specifically, the monitoring assembly is positioned away from RF power sources that can arbitrarily couple power into the monitoring assembly. Additionally, the line-of-sight feature of the subject apparatus simplifies the overall design and allows retrofitting of chambers not previously using such in-situ monitoring devices. The apparatus further provides versatility since the removable window allows interchanging different types of monitoring apparatus. Specifically, plasma-based lamps, laser interferometers, X-ray emitters and the like are optimized by selecting different types of window material (i.e., sapphire, quartz and the like) through which monitoring beams propagate. 
         [0038]    While the present invention has been described in considerable detail with reference to certain preferred versions, many other versions should be apparent to those of ordinary skill in the art. For example, other configurations of the process monitoring assembly should be apparent to those of ordinary skill in the art. In addition, the assembly may be used in other types of chambers than those used to illustrate the invention. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.