Patent Publication Number: US-11380517-B2

Title: System and method for spatially resolved optical metrology of an ion beam

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. Non-Provisional patent application Ser. No. 16/185,984 filed on Nov. 9, 2018, entitled “SYSTEM AND METHOD FOR SPATIALLY RESOLVED OPTICAL METROLOGY OF AN ION BEAM,” and incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present embodiments relate to semiconductor device surface treatments, and more particularly, to systems and methods for spatially resolved optical metrology of an ion beam. 
     BACKGROUND OF THE DISCLOSURE 
     Plasma etchers utilize directed ribbon ion beams to achieve complex etching and surface processing, thus solving many challenges in the fabrication of most advanced 3D semiconductor structures. Characterization of the ion beam, monitoring process end-points, and advanced plasma probing are relevant metrologies necessary for implanter operations. Currently, the metrologies are achieved with different mechanisms, such as Faraday cup arrays and free space optical emission spectrum (OES). These approaches have their limits and disadvantages. For example, the Faraday cup array is an in-chamber metrology device. As a result, the Faraday cup array presentation brings in extra particles and metals. Furthermore, patch charges accumulated on the shield of the Faraday cup array may also perturb the beam path and give results deviating from the actual values. The free space OES is widely adopted in process endpoint control, yet cannot resolve the ion beam&#39;s spatial distribution, and may suffer from various optical noises from the environment and from different materials in the etching beam. 
     The present disclosure addresses at least the above identified deficiencies of the prior art. 
     SUMMARY OF THE DISCLOSURE 
     The Summary of the Disclosure is provided to introduce a selection of concepts in a simplified form. The Summary of the Disclosure is not intended to identify key features or essential features of the claimed subject matter, nor intended as an aid in determining the scope of the claimed subject matter. 
     In an example embodiment, a system may include a chamber containing an ion source operable to deliver an ion beam to a wafer, and an optical collection module operable with the chamber, wherein the optical collection module includes an optical device for measuring a light signal of a volume of the ion beam. The system may further include a detection module operable with the optical collection module, the detection module comprising a detector for receiving the measured light signal and outputting an electric signal corresponding to the sampled volume of the ion beam. 
     In another example embodiment, a system for spatially resolved optical metrology of an ion beam may include a chamber containing an ion source operable to deliver an ion beam to a wafer, wherein the ion beam is a ribbon ion beam. The system may further include an optical collection module operable with the chamber, the optical collection module including an optical device, and a first plate adjacent the optical device, the first plate having a first aperture for receiving a sample of the ion beam. The optical collection module may further include a second plate adjacent the first plate, the second plate having a second aperture for receiving a light signal measured from a sample of the ion beam from the first plate. The system may further include a detection module operable with the optical collection module, the detection module comprising a detector operable to receive the light through second plate, and output an electric signal corresponding to the sample of the ion beam. 
     In another example embodiment, a method may include providing a chamber containing an ion source delivering an ion beam to a wafer, and measuring a light signal of a portion of the ion beam using an optical collection module operable with the chamber. The optical collection module may include an optical device adjacent the chamber, and a first plate adjacent the optical device, the first plate having a first aperture receiving the light signal. The optical collection module may further include a second plate adjacent the first plate, the second plate having a second aperture receiving the light signal through the first plate. The method may further include receiving the light signal at a detection module, and outputting an electric signal from the detection module, the electric signal corresponding to the measured light signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a system for spatially resolved optical metrology of an ion beam, in accordance with embodiments of the present disclosure. 
         FIG. 2  is a diagram of an optical collection module of the system of  FIG. 1 , in accordance with embodiments of the present disclosure. 
         FIG. 3  demonstrates operation of a set of aperture plates of the optical collection module of  FIG. 2 , in accordance with embodiments of the present disclosure. 
         FIG. 4  depicts an arrangement of two imaging devices for sampling a horizontal and a cross-section of the ion beam, in accordance with embodiments of the present disclosure. 
         FIG. 5  is a diagram illustration addition of a narrow band light source to collect absorptive optical signals, in accordance with embodiments of the present disclosure. 
         FIGS. 6A-6B  depict a plurality of scans performed to produce a corresponding number of intensity curves at different z locations, in accordance with embodiments of the present disclosure. 
         FIGS. 7A-7B  demonstrate assisted beam profiling with wafer pattern, in accordance with embodiments of the present disclosure. 
         FIG. 8  demonstrates an approach for retrieving a beam profile by spatial resolved optical imaging, in accordance with embodiments of the present disclosure. 
         FIG. 9  is a diagram of a detection module of the system of  FIG. 1 , in accordance with embodiments of the present disclosure. 
         FIG. 10  is a diagram of an optical collection module based on a charge-coupled device (CCD) array or equivalent 2D image devices, in accordance with embodiments of the present disclosure. 
         FIG. 11  is a flowchart illustrating an exemplary method according to the present disclosure 
     
    
    
     The drawings are not necessarily to scale. The drawings are merely representations, not intended to portray specific parameters of the disclosure. The drawings are intended to depict exemplary embodiments of the disclosure, and therefore are not to be considered as limiting in scope. In the drawings, like numbering represents like elements. 
     Furthermore, certain elements in some of the figures may be omitted, or illustrated not-to-scale, for illustrative clarity. The cross-sectional views may be in the form of “slices”, or “near-sighted” cross-sectional views, omitting certain background lines otherwise visible in a “true” cross-sectional view, for illustrative clarity. Furthermore, for clarity, some reference numbers may be omitted in certain drawings. 
     DETAILED DESCRIPTION 
     Systems and methods in accordance with the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, where embodiments of the methods are shown. The systems and methods may be embodied in many different forms and are not to be construed as being limited to the embodiments set forth herein. Instead, these embodiments are provided so the disclosure will be thorough and complete, and will fully convey the scope of the system and method to those skilled in the art. 
     As further described herein, provided is a spatial resolved optical system and method of using, the system including a precisely controlled scanning mechanism and a high sensitivity detection scheme to achieve non-invasive beam metrology, and to improve endpoint control. The embodiments herein provide a comprehensive solution for multiple problems without the current disadvantages of the prior art described above. 
     In some embodiments, during an etching process, optical signals can be generated from spontaneous emissions from excited ions and neutrals. The ions are mainly from the plasma source. The neutrals may come from the etchant gases, etch products, and sputtered material from wafer surface and chamber surfaces. By selecting the wavelength as appropriate, these components may be monitored separately for different purposes. When spatially resolved and correlated with scanning position, these optical signal intensities can reveal various distributions. 
     In some embodiments, spatial resolution may be achieved by optical filtering and precision mechanical scanning. The capacity allows fine resolve of plasma/neutral distribution by detected signature optical signals. The approach provides a non-invasive beam metrology method providing similar or better performance than current Faraday cup array solutions, yet without the disadvantage of in-vacuum contaminations. Approaches herein can be used to improve signal noise ratio in various OES applications, and to resolve material/beam exposure distribution on the wafer and chamber surface. A setup with additional narrow band light source illuminating the plasma area is suitable for absorption spectroscopy used to selectively detect certain components in the ion beam by their spectrum lines. 
     With the embodiments herein, multiple band/wavelengths can be monitored and correlated for various applications. For example, the correlation of the attenuation of one signal with the intensification of the other may be used for endpoint control in process. To further increase the optical signal&#39;s intensity, other methods can be used, namely, adding an inert gas with known spectral signature to the plasma source. The inert gas increases the strength of the optical signal around the ion ribbon beam, and emerges the ion beam in a higher neutral pressure. The higher pressure of the inert gas increases the collision between ions and neutrals, thus increasing the light intensity. 
     Although non-limiting, at least the following applications may benefit from the systems and methods of the present disclosure. In a first application, the high spatial resolution and high sensitivity of the systems and methods herein offer a noninvasive beam metrology method to resolve beam angle, spreading and evenness. The high spatial resolution can be used to distinguish different material/beam-exposure on wafer and chamber surfaces. The capacity is useful for some metrology schemes. 
     In a second application, systems and methods herein may increase the signal noise ratio for OES applications. The approaches herein focus on the optical signal from a small plasma slice, filtering out lights from the environment, source glowing, IR heating scattering, etc., thus providing a more relevant and accurate optical signal for process monitoring and control. 
     In a third application, the systems and methods herein offer the potential for more advanced plasma metrology, as a well confined collection angle is suitable for absorption spectroscopy. With narrow band coherent sources, certain components of the ion beam can be selectively detected. Other optical techniques, such as modulating of excitation and photon correlation, can be used to reduce noise and retrieve more plasma optical properties. 
     In a fourth application, the systems and methods herein offer an alternative detection scheme to address the relatively weak signal. For example, a no-loss dichroic mirror system may be used to divide the original emission into multiple bands with little loss, further filtered with narrow or broad band filters. The selectivity of the optical signal is mapped into the selectivity of the materials and particle species. Multiple signals can be correlated to make better decisions on processing endpoints. Methods herein may achieve a much higher sensitivity than spectrum meter in the current OES system with either high sensitivity photo diodes, or low noise photo multiply tubes, while also greatly cutting the bandwidth necessary for data communication and the storage size for data logging. 
     Turning now to  FIG. 1 , a system  100  for spatially resolved optical metrology of an ion beam according to various embodiments of the disclosure will be described. As shown, the system  100  include an interworking group of modules, such as an optical collection module  102 , a scanning module  104 , a detection module  106 , and a control module  108 . The optical collection module  102  operates with a chamber  110  containing an ion source  112  configured to deliver an ion beam  115  to a wafer  116 . The ion beam  115  may be extracted through an extraction aperture (not shown) as a ribbon ion beam having a beam width greater than a beam height. 
     During use, ions  118  may be in excited states and emit photons when passing the gap between the ion optics and the wafer  116 . The ions  118  may impact a surface  117  of the wafer  116  and excite sputtered atoms/molecules, wherein the latter may emit photons. Ions may recombine with electrons and emit photons in relatively rare situations, since there are very few electrons in the ion beam  115 . 
     Neutral gas molecules  120  may be present in many locations within the chamber  110 , and radicals may emit photons when passing the gap. The ions  118  may collide with the neutral gas molecules  120  and excite the neutral gas molecules  120  to emit photons. In some cases, the collision probability is &gt;0.1. In some embodiments, an additional inert gas can be added to ion source  112  as a signal enhancer. 
     As shown, the optical collection module  102  may include, or be mounted on, a mechanical device  123 . The mechanical device may be a moveable platform operable to scan the optical collection module  102  in multiple directions. For example, the mechanical device  123  has the capacity to scan in x and/or y and/or z directions. The precise coordinates for the mechanical device  123  can be set and read by the control module  108 . 
     Turning now to  FIG. 2 , the optical collection module  102  according to embodiments of the present disclosure will be described in greater detail. As shown, the optical collection module  102  is operable with the chamber  110  and may include an optical device  122  for measuring one or more light signals  121  of a volume, sample, or portion  124  of the ion beam  115 . In some embodiments, the optical device  122  is a telescope or equivalent optics. The optical collection module  102  may further include a first plate  126  having a first aperture  128  receiving the portion  124  of the ion beam  115 . The optical collection module  102  may further include a second plate  130  having a second aperture  132  receiving the portion  124  of the ion beam  115  from the first plate  126 . The optical signal representing the portion  124  of the ion beam  115  may then be transmitted to a detector  135  of the detection module  106 . 
     The optical collection module  102  may measure the light signals  121  instead of directly measuring a plasma  119 , the existing method of beam profiling. The light signals  121  are generated by a small resolved volume of the plasma  119 . Thus, when the light signals  121  are measured, properties of the sampled plasma  119  volume may be determined indirectly. As will be described in greater detail below, the light signals  121  may be measured by the detector  135 , such as a photomultiplier tube (PMT), a photo diode, or a charge-coupled device (CCD). In non-limiting embodiments, the optical collection module  102  achieves spatial resolution in the y-z directions shown. The optical device  122  may include a first convex lens  136  and a second convex lens  137 , wherein f 1  and f 2  represent the effective focal length of the respective lenses  136 ,  137 . The first and second apertures  128  and  132  accepts a narrow angle, and the telescope  122  further reduce the angle to: 
               ϕ   =       d   D     ⁢       f   2       f   1           ,         
and y-z resolution is determined as δ yz =gϕ, where g is the distance between the plasma source and the first convex lens  136 . In  FIG. 2 , δ y , δ z  represents the resolution in y and z directions.
 
     Turning now to  FIG. 3 , spatial resolution in the y-z directions achieved by the dual-aperture configuration of the first and second plates  126 ,  130  according to embodiments of the present disclosure will be described in greater detail. As shown, without the first plate  126  and the first aperture  128 , light from multiple sources S 1 , S 2 , S 3  will illuminate the same area, thus resulting in no spatial resolution. With the first aperture  128  alone, a portion  140  of light from adjacent sources will still leak into the detection, thus increase the detection noise. When the first aperture  128  is combined with the second aperture  132 , the second plate  130  reject lights from adjacent sources, and accepts light  142  just from the sources (e.g., S 2 ) along an axis extending through the first and second apertures  128 ,  132 . 
     In one non-limiting example, using practical dimensions of d=0.5 mm, D=50 mm, 
                   f   2       f   1       =     0   .   2       ,         
ϕ=2×10 −3 , the corresponding spatial resolution is 0.5 m×ϕ=1×10 −3  m=1 mm. In some examples, higher resolution can be achieved with smaller apertures and a larger telescope magnification.
 
     As shown in  FIG. 4 , to measure a beam evenness of the ion beam  115 , the system  100  may further include one or more imaging devices  144  operable to generate an image of the ion beam  115  as the ion beam  115  scans the wafer  116 . In some embodiments, the imaging devices  144  include a highly sensitive camera with suitable filter placed above a top window  146  of the chamber  110  for directly imaging the ion beam  115 . The image corresponds to the beam distribution of the ion beam  115  across the x direction, and can be used as a measurement of ion beam evenness along the x direction. In some embodiments, the different collection directions from the multiple imaging devices  144  may resolve all x, y, z dimensions of the ion beam. 
     Turning now to  FIG. 5 , an optical collection module  202  according to embodiments of the present disclosure will be described in greater detail. The optical collection module  202  may be the same or similar to the optical collection module  102  shown in  FIG. 2  and described above. In the embodiment shown, the optical collection module  202  may include a light source  246  positioned external to the chamber  210 . The light source  246  may be operable to deliver a light into the chamber  210  for detection by the detector  235  via the optical device  222  and the first and second apertures  228 ,  232 . 
     More specifically, light from the light source  246  may be introduced from the opposite side of the imaging window to form an absorption spectroscopy setup. When the light from the light source  246  is absorbed by the particles (e.g., ions, molecules and radicals) along the light path, a shadow is cast on the detection optics. The distribution of the shadow indicates the distribution of the absorbers. In various non-limiting embodiments, the light source  246  can be a broadband classical source, or a narrow band coherent source. The latter can selectively image certain particles with transition resonant with the source frequency. Modulation can be added to the source to suppress the noise, similar to a lock-in amplifier. 
       FIGS. 6A-6B  depict a plurality of scans performed to produce a corresponding number of intensity curves, in accordance with embodiments of the present disclosure. With the optical intensity of selected lines and the scan coordinates, the plasma/neutral density within the chambers  110 ,  210  can be mapped out. For example, when scanning along the Y-direction at different Z distances (e.g., Z 1 , Z 2 , Z 3 ) from the wafer  316 , as shown in  FIG. 6A , the ion beam profile can be mapped out, as shown as Z 1 , Z 2 , and Z 3  in  FIG. 6B . Z 1 , Z 2 , and Z 3  in  FIG. 6B  represent three intensity curves, interpolated to get beam angle and a beam spread of the ion beam. In other embodiments, when the scan is along the X-direction, the evenness of the ion beam across the wafer  316  can be mapped out. When the scan is along the Y direction, the ion beam cross section can be mapped out. In yet other embodiments, if the optical collection module is fixed, a small well confined area of the wafer is monitored. The optical signal from the confined area can be used as a low noise signal for end-point control. 
       FIGS. 7A-7B  demonstrate assisted beam profiling with wafer pattern, in accordance with embodiments of the present disclosure. In the non-limiting embodiment shown, a scan of the ion beam  415  may be performed along the y-direction ( FIG. 7A ) close to the surface  417  of the wafer  416 , and then mapped to produce the intensity curve  421  shown in  FIG. 7B . In some embodiments, the spatial resolution will resolve the optical intensity distribution generated by different wafer materials/beam exposures. 
       FIG. 8  demonstrates an approach  500  for retrieving a beam profile by spatial resolved optical imaging, in accordance with embodiments of the present disclosure. In a first process  501 , an image  502  of an ion beam is produced, e.g., by a digital camera. In a second process  503 , a slice  504  of the image  502  is processed to retrieve the blue component of RGB. In a third process  505 , the blue component is mapped to retrieve an intensity distribution  506 . In a fourth process  507 , the intensity distribution  506  may be fit with a line model  508  to determine the beam intensity peak locations. In a fifth process  509 , the line model  508  may be overlapped with the original image  502  to verify accuracy. 
       FIG. 9  is a diagram of the detection module  106  of the system of  FIG. 1 , in accordance with embodiments of the present disclosure. As shown, the detection module  106  may include a beam splitting apparatus  152  for splitting the sampled portion  124  of the ion beam  115  into a plurality of bands  153 ,  154 . In one non-limiting embodiment, the beam splitting apparatus  152  is a plurality of dichroic mirrors used to avoid loss. The detection module  106  may further include one or more filters  156  receiving the bands  153 ,  154 . In non-limiting embodiments, the filter  156  is a narrow band filter tapered to the spectrum. The filter  156  is used to select the interested optical signals and cut off the noise. 
     The detection module  106  may further include the detector  135 , wherein the detector  135  is operable to receive one of the bands from the filter  156 . In various non-limiting embodiments, the detector  135  may be one of: a photodiode, a photo multiplier tube, and an avalanche photo detector. The highly sensitive detector  135  may turn optical intensity into electric/digital signals  160  for further processing. In one embodiment, the electric signal  160  may be sent to either a counter or analog-to-digital converter (ADC) to be digitalized before being sent to the control module  108 . 
     Referring again to  FIG. 1 , the control module  108  according to embodiments of the present disclosure will be described in greater detail. As shown, the control module  108  is operable with the optical collection module  102  and the detection module  106 . In some non-limiting embodiments, the control module includes a processing device  164  operable to receive the electric signal  160  from the detection module  106 . The processing device  164  is further operable to process the electric signal  160 , or the digitized electric signal, to determine at least one of the following: an evenness of the ion beam  115  across the wafer  116 , a cross-section of the ion beam  115 , and a profile of the ion beam  115 . In some embodiments, as described above, the processing device  164  may determine the profile of the ion beam by performing a plurality of scans along a first axis parallel to the surface  117  of the wafer  116 , wherein each of the scans is performed at a different z-distance normal to the surface  117  of the wafer  116 . The processing device may further generate a plurality of intensity curves for the plurality of scans, and then interpolate the plurality of intensity curves to determine a beam angle and a beam spread of the ion beam. 
     The processing device  164  may further coordinate the optical signals and the scanning module  104 . The processing device  164  can either set the coordinate and read the optical signal, or simply park the scanning module  104  and constantly monitor the signal. The processing device  164  can also control the injecting light source (e.g., frequency, amplitude, or their modulations) to retrieve various information, or to increase signal noise ratio (e.g., by adding an amplitude modulation to increase SNR as similar in a lock-in amplifier). By correlating the control module  108  and the scanning module  104 , comprehensive metrology data can be collected for multiple purposes etc. 
       FIG. 10  is a diagram of an alternative optical collection module  602 , in accordance with embodiments of the present disclosure. As shown, the optical collection module  602  may include the chamber  610  containing the ion source  612  operable to deliver the ion beam  615  to the wafer  616 . The ion beam  615  may be extracted through an extraction aperture (not shown) as a ribbon ion beam having a beam width greater than a beam height. 
     As shown, the optical collection module  602  is operable with the chamber  610  and may include an optical device  622  for sampling the light signals generated by a volume or portion  624  of the ion beam  615 . In some embodiments, the optical device  622  is a telescope or equivalent optics. The optical collection module  602  may further include a linear array  670 , such as a PMT array to replace one scan dimension, or a CCD array to replace the two-dimensional scan. In some embodiments, where a 2-D comb structure is provided in front of a CCD, or a 1-D comb structure in front of a line CCD/PMT array, the scanning structure may be partially or completely replaced (when the sensitivity allows). Omitting the mechanical scans allow faster measurement with one shot (1×τ exposure ) or an one dimensional scan (n×τ exposure ), versus a two dimensional scan (n×n×τ exposure ). 
       FIG. 11  is a flowchart illustrating an exemplary method  700  according to the present disclosure. In block  701 , the method  700  may include providing a chamber containing a plasma source delivering an ion beam to a wafer. In some embodiments, the ion beam may be a ribbon ion beam. In block  703 , the method  700  may include measuring a light/optical signal generated from a portion of the ion beam using an optical collection module operable with the chamber. In some embodiments, the optical collection module includes an optical device adjacent the chamber, a first plate adjacent the optical device, wherein the first plate has a first aperture receiving the light signal. The optical collection module may further include a second plate adjacent the first plate, the second plate having a second aperture receiving the light signal from the first plate. 
     At block  705 , the method  700  may include receiving the light signal at a detection module. In some embodiments, the light signal is spatially resolved and then used to rebuild the ion beam&#39;s spatial distribution. 
     In some embodiments, the detection module includes at least one beam splitting apparatus for splitting the light signal of the portion of the ion beam into a plurality of bands, a filter receiving a first band of the plurality of bands, wherein the detector receives the first band of the plurality of bands from the filter. 
     In block  707 , the method  700  may include outputting an electric signal from the detection module, the electric signal corresponding to the light signal of the portion of the ion beam. In some embodiments, the electric signal may be sent to either a counter or analog-to-digital converter (ADC) to be digitalized before being sent to the control module. 
     At block  709 , the method  700  may include processing the electric signal to determine at least one of the following: an evenness of the ion beam across the wafer, a cross-section of the ion beam, and a profile of the ion beam. In some embodiments, the profile of the ion beam is determined by performing a plurality of scans along a first axis parallel to a surface of the wafer, wherein the plurality of scans is performed at differing distances normal to the surface of the wafer, and generating a plurality of intensity curves for the plurality of scans. The profile is further determined by interpolating the plurality of intensity curves to determine a beam angle and a beam spread of the ion beam. 
     In some embodiments, the method  700  may further include generating one or more optical signals of the portion of the ion beam from emissions of a plurality of excited ions and of a plurality of neutral gas molecules. The method  700  includes further monitoring at least one of the plurality of excited ions and the plurality of neutral gas molecules at a given wavelength, and mapping a density of at least one of the plurality of excited ions and the plurality of neutral gas molecules. 
     In some embodiments, the method  700  may further include delivering a light into the chamber for detection by the detection module, wherein the light is absorbed by the plurality of excited ions and the plurality of neutral gas molecules. A distribution of light, or lack thereof (e.g., shadows), generated on the optical device as a result of the light being absorbed by the plurality of excited ions and the plurality of neutral gas molecules may then be generated. 
     The foregoing discussion has been presented for purposes of illustration and description and is not intended to limit the disclosure to the form or forms disclosed herein. For example, various features of the disclosure may be grouped together in one or more aspects, embodiments, or configurations for the purpose of streamlining the disclosure. Various features of the certain aspects, embodiments, or configurations of the disclosure may be combined in alternate aspects, embodiments, or configurations. Moreover, the following claims are hereby incorporated into the Detailed Description by reference, with each claim standing alone as a separate embodiment of the present disclosure. 
     Embodiments herein may be computer implemented. For example, the processing device  164  may include a computer processor to perform logic operations, computational tasks, control functions, etc. In some embodiments, the computer processor may be a component of a processor. The computer processor may include one or more subsystems, components, modules, and/or other processors, and may include various logic components operable using a clock signal to latch data, advance logic states, and synchronize computations and logic operations. During operation, the computer processor may receive signals transmitted over a LAN and/or a WAN (e.g., T1, T3, 56 kb, X.25), broadband connections (ISDN, Frame Relay, ATM), wireless links (802.11, Bluetooth, etc.), and so on. In some embodiments, the signals may be encrypted using, for example, trusted key-pair encryption. Different systems may transmit information using different communication pathways, such as Ethernet or wireless networks, direct serial or parallel connections, USB, Firewire®, Bluetooth®, or other proprietary interfaces. (Firewire is a registered trademark of Apple Computer, Inc. Bluetooth is a registered trademark of Bluetooth Special Interest Group (SIG)). 
     In general, the computer processor executes computer program instructions or code stored in a memory unit and/or storage system. For example, when executing computer program instructions, the computer processor causes the processing device  164  to receive inputs, such as any of the optical signals parameters discussed herein, and provide, from the computer processor, the outputs. 
     While executing computer program code, the computer processor can read and/or write data to/from the memory unit and/or the storage system (not shown). The storage system may comprise VCRs, DVRs, RAID arrays, USB hard drives, optical disk recorders, flash storage devices, and/or any other data processing and storage elements for storing and/or processing data. Although not shown, the processing device  164  could also include I/O interfaces communicating with one or more hardware components of computer infrastructure to enable a user to interact with the processing apparatus  200  (e.g., a keyboard, a display, camera, etc.). 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” is understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments also incorporating the recited features. 
     The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Accordingly, the terms “including,” “comprising,” or “having” and variations thereof are open-ended expressions and can be used interchangeably herein. 
     The phrases “at least one”, “one or more”, and “and/or”, as used herein, are open-ended expressions and are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “one or more of A, B, or C” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. 
     All directional references (e.g., proximal, distal, upper, lower, upward, downward, left, right, lateral, longitudinal, front, back, top, bottom, above, below, vertical, horizontal, radial, axial, clockwise, and counterclockwise) are just used for identification purposes to aid the reader&#39;s understanding of the present disclosure, and do not create limitations, particularly as to the position, orientation, or use of the disclosure. Connection references (e.g., attached, coupled, connected, and joined) are to be construed broadly and may include intermediate members between a collection of elements and relative movement between elements unless otherwise indicated. As such, connection references do not necessarily infer two elements are directly connected and in fixed relation to each other. 
     Furthermore, identification references (e.g., primary, secondary, first, second, third, fourth, etc.) are not intended to connote importance or priority, and are used to distinguish one feature from another. The drawings are for purposes of illustration, and the dimensions, positions, order and relative sizes reflected in the drawings attached hereto may vary. 
     Furthermore, the terms “substantial” or “approximately,” as well as the terms “approximate” or “approximately,” can be used interchangeably in some embodiments, and can be described using any relative measures acceptable by one of ordinary skill in the art. For example, these terms can serve as a comparison to a reference parameter, to indicate a deviation capable of providing the intended function. Although non-limiting, the deviation from the reference parameter can be, for example, in an amount of less than 1%, less than 3%, less than 5%, less than 10%, less than 15%, less than 20%, and so on. 
     Still furthermore, although the illustrative method  700  is described above as a series of acts or events, the present disclosure is not limited by the illustrated ordering of such acts or events unless specifically stated. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein, in accordance with the disclosure. In addition, not all illustrated acts or events may be necessary to implement a methodology in accordance with the present disclosure. Furthermore, the method  700  may be implemented in association with the formation and/or processing of structures illustrated and described herein as well as in association with other structures not illustrated. 
     In view of the foregoing, at least the following technical benefits and advantages are achieved by the embodiments disclosed herein. Firstly, embodiments herein improve current tool design to fix some critical problems, such as the inaccuracy and metal contamination brought by in-vacuum beam metrology systems. Secondly, embodiments herein improve the OES process monitoring capacity, opening up potential for future advanced optical metrologies. Thirdly, the system can be easily adapted for any plasma etchers, and fast deployment can be achieved with no major modification of the existing systems, thus minimizing downtime. 
     While certain embodiments of the disclosure have been described herein, the disclosure is not limited thereto, as the disclosure is as broad in scope as the art will allow and the specification may be read likewise. Therefore, the above description is not to be construed as limiting. Instead, the above description is merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.