Patent Publication Number: US-2021193444-A1

Title: Normal-incidence in-situ process monitor sensor

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/051,082 filed on Jul. 31, 2018. The entire content of the above-identified application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to in-situ etch process monitoring, and, more particularly, to methods, systems, and apparatuses for real-time in-situ film properties monitoring of the plasma etch process. 
     Plasma etch processes are commonly used in conjunction with photolithography in the process of manufacturing semiconductor devices, liquid crystal displays (LCDs), light-emitting diodes (LEDs), and some photovoltaics (PVs). 
     In many types of devices, such as semiconductor devices, a plasma etch process is performed in a top material layer overlying a second material layer, and it is important that the etch process be stopped accurately once the etch process has formed an opening or pattern in the top material layer, without continuing to etch the underlying second material layer. The duration of the etch process has to be controlled accurately so as to either achieve a precise etch stop at the top of an underlying material, or to achieve an exact vertical dimension of etched features. 
     For purposes of controlling the etch process various methods are utilized, some of which rely on analyzing the chemistry of a gas in a plasma processing chamber in order to deduce whether the etch process has progressed, for example, to an underlying material layer of a different chemical composition than the material of the layer being etched. 
     Alternatively, in-situ metrology devices (optical sensors) can be used to directly measure the etched layer during the etch process and provide feedback control for accurately stopping the etch process once a certain vertical feature has been attained. For example, in a generic spacer application the goal for an in-situ optical sensor for film thickness monitoring is to stop anisotropic oxide-etch at a few nm before touchdown (soft landing), then switch to isotropic etching to achieve an ideal spacer profile. Further, the in-situ metrology devices may be used for real-time actual measurement of the films and etch features during the etch process to determine information about the sizes of structures which can be used to control the etch process and/or to control subsequent processes (e.g., a process to compensate for a certain out-of-specification dimension). 
     The foregoing “Background” description is for the purpose of generally presenting the context of the disclosure. Work of the inventor, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention. 
     SUMMARY 
     An aspect of the present disclosure includes an apparatus for in-situ etching monitoring in a plasma processing chamber. The apparatus includes a continuous wave broadband light source, an illumination system configured to illuminate an area on a substrate with an incident light beam being directed at normal incidence to the substrate, a collection system configured to collect a reflected light beam being reflected from the illuminated area on the substrate, and direct the reflected light beam to a detector, and processing circuitry. The processing circuitry is configured to process the reflected light beam to suppress (e.g., filter or subtract) background light, determine a property (e.g., thickness) of the substrate or structures formed thereupon, based on reference light beam and the reflected light beam, and control an etch process based on the determined property. 
     Another aspect of the present disclosure includes a plasma processing system. The system includes a plasma processing chamber and a normal incidence reflectometer with zero degree AOI (angle of incidence). The normal incidence reflectometer includes a continuous wave broadband light source, a detector, an illumination system configured to illuminate an area on a substrate disposed in the plasma processing chamber with an incident light beam being directed at normal incidence to the substrate, a collection system configured to collect a reflected light beam being reflected from the illuminated area on the substrate, and direct the reflected light beam to the detector, and processing circuitry. The processing circuitry is configured to process the reflected light beam to suppress background light, determine a property of the substrate or structures formed thereupon based on reference light beam and the reflected light beam that are processed to suppress the background light, and control an etch process based on the determined property. 
     Yet another aspect of the present disclosure includes a method for in-situ etch monitoring. In the disclosed method, incident light beam is directed at normal incidence to a substrate disposed in a plasma processing chamber, and the incident light generates an illuminated area on surface of the substrate. In addition, a portion of the incident light beam is split to a detector to collect a reference light beam. Background light generated from the plasma and a reflected light beam is also collected from the illuminated area. Further, the reflected light beam is processed to suppress the background light. A property of the substrate or structures formed thereupon is determined based on the reference light beam and the reflected light beam by using an algorithm or a reference library, and the etch process is controlled based on the determined property. 
     The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a system for etch process monitoring in accordance with some embodiments. 
         FIG. 2  is a schematic diagram of an exemplary optics module in accordance with some embodiments. 
         FIG. 3A  is a schematic view of a first exemplary configuration to obtain a reference beam in accordance with some embodiments. 
         FIG. 3B  is a schematic view of a second exemplary configuration to obtain a reference beam in accordance with some embodiments. 
         FIG. 3C  is a schematic view of a third exemplary configuration to obtain a reference beam in accordance with some embodiments. 
         FIG. 3D  is a schematic view of a fourth exemplary configuration to obtain a reference beam in accordance with some embodiments. 
         FIG. 4A  is a block diagram of an optical modulation/shutter module in accordance with some embodiments. 
         FIG. 4B  is a schematic that shows a timing diagram of a shutter in accordance with some embodiments. 
         FIG. 5  is a flowchart that shows a method for in-situ monitoring of an etch process in accordance with some embodiments. 
         FIG. 6  is a schematic that shows exemplary results. 
         FIG. 7  is an exemplary block diagram of a controller in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout several views, the following description relates to a system and associated methodology for real-time in-situ film properties monitoring of a plasma process of patterned or un-patterned wafer in semiconductor manufacturing. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment, but do not denote that they are present in every embodiment. Thus, the appearances of the phrases “in one embodiment” in various places through the specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. 
       FIG. 1  is a side view schematic of a plasma processing system  100  equipped with an optical sensor  101  according to one example. The plasma processing system  100  further includes a plasma processing chamber  124 . 
     The optical sensor  101  can be a normal incidence reflectometer with zero degree angle of incidence (AOI) that includes an optics module  102  (illumination and collection), a light source  104 , a shutter  106 , spectrometer  112  and a controller  114 . The optical sensor  101  generates an incident light beam  120  from the light source  104  and receives a reflected light beam  122  for analysis. The incident light beam  120  and the reflected light beam  122  propagate along the normal to the substrate  116  in the plasma processing chamber  124 . The optics module  102  further includes an illumination system  108  and a collection system  110 . The optical sensor  101  is configured for measuring the reflected light beam  122  from an illuminated area  118  on a substrate  116  during a plasma etching process in the plasma processing chamber  124 . The illuminated area  118  may be adjustable as a function of the size of the substrate  116 . In one embodiment, the optics module  102  may be located outside of the plasma processing chamber  124 . In another embodiment, the optics module  102  can be installed in the plasma processing chamber. As shown in  FIG. 2 , the optics module  102  can be installed inside a tube, and the tube is made of stainless steel or aluminum alloy and inserted into the plasma processing chamber  124  through a top wall of the plasma processing chamber  124 . 
     In the optical sensor  101 , the light source  104  is used to form the incident light beam  120  for substrate illumination. In an embodiment, the light source  104  is a broadband light source such as continuous wave (CW) broadband light source, for example a laser driven plasma light source (LDLS) that provides light with very high brightness across a broad spectrum UV (ultraviolet)-Vis (visible)-NIR (near infrared) (i.e., 190 nm-2000 nm) with a long-life bulb (&gt;9000 hours) such as EQ-99X LDLS™ from ENERGETIQ. In one embodiment, the light source  104  may be fiber coupled to the illumination system  108  after being modulated by an optional shutter  106 . In another embodiment, the light source  104  may be fiber coupled to the illumination system  108  directly without passing through the shutter  106 . 
     The light source  104  may or may not be mounted proximate to the plasma processing chamber  124  or any enclosure housing the optical sensor  101 , and in the case of being mounted remotely, the incident light beam  120  can be fed into other components proximate to the plasma processing chamber  124  by an optical fiber, or by a set of optical components such as mirrors, prisms, and lenses as described later herein. The optical sensor  101  may also include relay optics and polarizers for the incident and reflected light beams. In one example, the relay optics use parabolic mirrors to direct the beams and minimize optical aberrations. 
     The incident light beam  120  is being reflected from the illuminated area  118  on substrate  116  to form the reflected light beam  122 . The optical sensor  101  also includes a detector such as spectrometer  112 . The spectrometer  112  can be a dual-channel broad-band high SNR (signal to ratio) spectrometer including a measurement channel (i.e., measurement spectrometer) for measuring the spectral intensity of the reflected light beam  122  and a reference channel (i.e., reference spectrometer) for measuring the spectral intensity of a reference light beam  126 . The measurement channel of spectrometer  112  may be fiber coupled to the collection system  110 . 
     Before the incident light beam  120  is directed at normal incidence to the substrate  116 , a portion of the incident light beam  120  is split to serve as the reference light beam  126  and the reference light beam  126  is subsequently directed to a reference channel of spectrometer  112  (i.e., reference spectrometer). The purpose of collecting the reference light beam  126  is to monitor the spectral intensity of the incident light beam  120  so any changes of the intensity of incident light beam  120  can be accounted for in the measurement process. Such changes of intensity may occur due to drifting output power of light source  104 , for example, which drift can be wavelength-dependent. In another implementation, the intensity of the reference light beam  126  may be measured by one or more photodiodes or the like. For example, a photodiode may detect the reference light beam and provide a reference signal that is proportional to the intensity of the incident light beam  120  which is integrated across the entire illumination spectrum (e.g., UV-VIS-NIR). 
     In one implementation, the intensity of the reference light beam  126  may be measured using a set of photodiodes. For example, the set of photodiodes may include three photodiodes, spanning UV-VIS-NIR wavelength respectively. A filter may be installed in front of each photodiode of the set of photodiodes. For example, band pass filters may be used to monitor a portion of the spectrum (e.g., UV, VIS, NIR) for intensity variation of the light source  104 . In one implementation, the reference light beam may be dispersed using a prism or a grating into the set of photodiodes. Spectrally-dependent intensity variation of the light source  104  may thus be tracked and corrected for without the use of a reference spectrometer. Exemplary configurations for obtaining the reference light beam are shown in  FIGS. 3A and 3B  discussed below. 
     The incident light beam  120  can be modulated by a chopper wheel or shutter  106  in order to account for the background light (i.e., light which is not indicative of the reflected light of the incident light beam  120  such as plasma light emission or equipment light in the plasma processing chamber) measured by the measurement channel of spectrometer  112  when the incident light beam  120  is blocked by the shutter  106 . 
     In another embodiment, the chopper wheel or the shutter  106  can be omitted in the optical sensor  101 . The incident light beam  120  can be fiber coupled to the illumination system  108  directly through the light source  104 . In such an embodiment, the background light due to the plasma light emission or equipment lights can be filtered out from the reflected light beam through signal processing algorithms. 
     The measured spectral intensity of the background light collected from the plasma processing chamber  124  when the shutter blocks the incident light, the measured spectral intensity of the reflected light beam  122  and the measured spectral intensity of the reference light beam  126  are provided to a controller  114 . The controller  114  processes the measured spectral intensity of the reflected light beam  122  to suppress the background light. For example, the controllers  114  can subtract the spectral intensity of the background light from the spectral intensity of the reflected light beam  122 . In addition, the measured spectral intensity of the reference light beam  126  can be analyzed by the controller  114  to monitor any intensity change of incident light beam  120  and the intensity changes of incident light beam  120  can be accounted for in the measurement process. The controller  114  uses special algorithms, such as a machine learning algorithm to determine a property or multiple properties for a layer of interest (e.g., feature dimension, optical properties), based on the reference light beam and the reflected light beam that are processed to suppress the background light to control the plasma etching process as described further below. 
     In another embodiment, the shutter  106  is not introduced in the optical sensor  101 , and the light source  104  may be fiber coupled to the illumination system  108  directly without passing through the shutter  106 . The controller  114  can use algorithms to calculate spectral intensity of the background light from the measured spectral intensity of the reflected light beam  122 . The controller  114  can further process the measured spectral intensity of the reflected light beam  122  to suppress (e.g., filter or substrate) the background light based on the calculated spectral intensity of the background light, or alternatively, if the interference from background light is sufficiently low, no correction for background illumination may be required. 
     The optical sensor  101  and associated methodologies can also use periodic measurements on a reference wafer (calibration), such as a bare silicon wafer, to compensate for optical sensor or etch chamber components drifts as described later herein. 
     Still referring to  FIG. 1 , the controller  114  is connected with the light source  104 , the shutter  106  and the spectrometer  112 . The controller  114  can acquire data from the light source  104 , the shutter  106  and the spectrometer  112 , and process the acquired data. The controller  114  can send instructions to the light source  104 , the shutter  106  and the spectrometer  112  according to processed data. 
       FIG. 2  is a schematic view of the optics module  102  according to one example. As shown in  FIG. 2 , the optics module  102  can be integrated inside and on top of a tube  222 . The tube  222  can be made of stainless steel, aluminum alloy, dielectric material, or the like. The tube  222  can be inserted into the plasma processing chamber  124  through an upper wall  240  of the plasma processing chamber. A bottom portion of the tube  222  may protrude through the upper wall  240 . The tube  222  can be positioned at the center of the top wall of the plasma processing chamber. The tube  222  can also be located off-center depending on the measurement requirements. The tube  222  can use a vacuum seal  226  and a vacuum seal flange  228  to be mounted against the upper wall  240 . The tube  222  can optionally include a gas supply pipe  224  that is connected with a side portion of the tube  222  and can be used to inject processing gas or a purge gas  236  to the plasma processing chamber  124  through gas injection holes  234 . The tube  222  can include a lower window  202  configured to prevent contamination from the plasma processing chamber to the inside of tube. In an embodiment, the lower window  202  can be perforated and the gas  236  can be allowed to escape from the lower window  202 . The tube  222  can also include an upper window  204 . The upper window  204  is configured to serve as a vacuum seal where a portion above the upper window  204 , of the tube  222  is under atmospheric pressure, and the other portion below the upper window  204 , of the tube, is under vacuum. The lower window  202  can be quartz, fused silica, or sapphire. The upper window can also be quartz, fused silica, or sapphire according to requirements which may include resistance of window material to aggressive chemistries in plasma processing chamber  124 , and the need to transmit needed wavelength, including in the deep UV part of the spectrum, for example. 
     The optics module  102  includes the illumination system  108  and the collection system  110 . As shown in  FIG. 2 , the illumination system  108  can include a first off-axis parabolic mirror  212 , a first polarizer  208 , and a beam splitter  206 . In some embodiments, the first off-axis parabolic mirror  212  is a 90° off-axis parabolic mirror. The collection system  110  can include a second off-axis parabolic mirror  220 , a second Rochon polarizer  216 , and a fold mirror  214 . In some embodiments, the second off-axis parabolic mirror  220  is another 90° off-axis parabolic mirror. In an exemplary operation, the incident light beam  120  is generated by the light source  104  and guided to the first off-axis parabolic mirror  212  through fiber  210 . The first off-axis parabolic mirror  212  can be mirrors coated with high-reflectance coatings, such as aluminum, gold, or the like. The first off-axis parabolic mirror  212  is configured to direct the incident light beam  120  and minimize optical aberrations. The incident light beam  120  is directed by the first off-axis parabolic mirror  212  to the first polarizer  208 . 
     The optional first polarizer  208 , if present, imposes a linear polarization to the incident light beam  120  that reaches the substrate  116 . The first polarizer  208  may be a Rochon Polarizer with high extinction ratio, large e- and o-ray separation, for example, a MgF2 Rochon polarizer, an Alpha-BBO Rochon Polarizer, or the like. Polarization of the incident light beam  120  increases the signal to noise ratio of the reflectometer signal, and thereby improves measurement accuracy and improves sensitivity of feature dimension measurements compared to an un-polarized incident light beam. 
     After passing through the first polarizer  208 , the incident light beam  120  reaches the beam splitter  206 . The beam splitter  206  can direct the incident light beam  120  at normal incidence toward the substrate  116  and generate the illuminated area  118 . The beam splitter  206  can further split a portion of the incident light beam  120  to form the reference light beam  126  and the reference light beam  126  is subsequently directed to the reference channel of the spectrometer  112  by other optical components, which is illustrated in  FIGS. 3A-3B . The beam splitter can be a cube made from two triangular glass prisms, a half-silvered mirror, or a dichroic mirrored prism, or the like. 
     The size of the illuminated area  118  on substrate  116  can vary from 50 microns to 60 mm (millimeters) or more. The shape of the illuminated area  118  can be circular, but may also be changed into a non-circular shape by the use of an aperture mask inserted into any of the incident light beam  120  or reflected light beam  122  (not shown). The size of the illuminated area  118  may depend on the sizes and characteristics of the structures being measured on the substrate  116  and may be adjustable to ensure good signal. The illuminated area  118  may cover multiple structures on the substrate  116 . Thus, the detected optical properties (e.g., index of refraction) may represent an average of the features associated with the many structures on the substrate  116 . 
     In an embodiment, the incident light beam  120  may be passed through an aperture (not shown) that is located prior to the first off-axis parabolic mirror  212 . The aperture may be modified to generate an illuminated spot having different shapes (e.g., rectangular, square). Subtle modification to the aperture can be used to efficiently optimize the size and shape of the illuminated area on the substrate, for example based on the sizes and characteristics of the structures being measured. 
     The incident light beam  120  is therefore reflected from the surface of the substrate  116  to generate the reflected light beam  122 . The reflected light beam  122  is passed through the lower window  202 , the upper window  204 , and the beam splitter  206 . It should be noted that the beam splitter  206  is designed to allow the propagation of the reflected light beam  122  with minimum signal loss. The reflected light beam  122  is then directed by the fold mirror  214  to the optional second Rochon Polarizer  216 . The second Rochon polarizer  216 , if present, is configured to only allow p-polarized light reflected from the substrate  116  to be measured. After passing through the second Rochon polarizer  216 , the reflected light beam  122  is passed through the second off-axis parabolic mirror  220 . After passing through the second off-axis parabolic mirror  220 , the reflected light beam  122  can be collected via optical fiber  218  and directed to the measurement channel of the spectrometer  112 . The optical fiber  218  is coupled to the measurement channel of the spectrometer  112 . The second off-axis parabolic mirror  220  may be similar to the first off-axis parabolic mirror  212 . In various embodiments of optical sensor  101 , no polarizers, or one or both optional polarizers  208  and  216  may be used, depending on the signal-to-noise requirements and other measurement requirements. 
     In further embodiments, the optical sensor  101  illustrated in  FIG. 2  can include other optical components, such as mirrors, prisms, lenses, spatial light modulators, digital micro-mirror devices, and the like, to steer the incident light beam  120  and the reflected light beam  122 . The configuration and component layout of the optical sensor  101  of  FIG. 2  does not necessary need to be as shown exactly in  FIG. 2 . By way of additional optical components, the light beams can be folded and steered to facilitate packaging the in-situ optical sensor into a compact packaging suitable for mounting on the wall of the plasma processing chamber  124 . 
       FIG. 3A  is a first exemplary configuration to obtain a reference light beam according to one example. From the shutter  106 , a portion of the light output can serve as the reference light beam  126  and can be directed by a mirror  302  into a reference channel of the spectrometer  112 . The reference light beam may be focused into the optical fiber using a lens  304 . 
       FIG. 3B  is a second exemplary configuration to obtain a reference light beam according to one example. The beam splitter  206  in the path of the incident light beam  120  can be used to direct a portion of the incident light beam into the reference channel of the spectrometer  112 . A prism  306  may be used to focus the reference light beam  126  into the optical fiber. In one implementation, the intensity of the reference light beam may be measured using one or more photo detectors (e.g., UV, Vis, NIR) connected to the controller  114  as discussed previously herein. 
       FIGS. 3C and 3D  provide a third and a fourth configurations respectively to measure the light intensity of the reference light beam directly at the light source output. In  FIG. 3C , a portion of the light output generated by the light source  104  can transmit through the shutter  106  that is optional and can be omitted, a lens  310 , a lens  312 , and received by an optical fiber. The optical fiber further guides the received light beam to the illumination system. In addition, a portion of the light output of the light source  104  can transmit through a lens  314 , and be received by another optical fiber. The other optical fiber can further be coupled with the reference channel of the spectrometer  112 . In  FIG. 3D , a portion of the light output of the light source  104  can transmit through a lens  314  and received by one or more photodiodes  318 . The photodiodes  318  further measures the intensity of the received reference light beam  126 . 
       FIG. 4A  is a block diagram of an optical modulation/shutter module according to one example. In one implementation, the shutter  106  may move back and forth between two positions to block or allow the incident light beam  120  into the plasma processing chamber  124 . The shutter  106  may include a stepper motor. The shutter  106  with a stepper motor provides high switching speed and high repeatability and reliability. The shutter  106  may be controlled via a shutter controller  400  synchronized with the spectrometer  112 . The data acquisition module  402  is connected to the reference channel of the spectrometer  112  and the measurement channel of the spectrometer  112 . In one implementation, the shutter  106  may be a continuous rotation optical chopper. 
       FIG. 4B  is a schematic that shows a timing diagram of the shutter  106  according to one example. The read out of the charged coupled device (CCD) has a clean cycle. When the shutter is open, the incident light beam  120  reaches the substrate  116  and thus, the measured light by the measurement channel of the spectrometer  112  is indicative of the reflected light beam  122  and the background light (e.g., plasma emission light). M cycles (i.e., CCD integration/data read) can be measured and averaged to improve signal to noise ratio (SNR). When the shutter is closed, the incident light beam  120  does not reach the substrate  116  and thus the light measured by the measurement channel of the spectrometer  112  is indicative of the background light (e.g., plasma emission light). N cycles (i.e., CCD integration/data read) can be measured and averaged to improve SNR. Thus, the controller  114  may process the collected intensities (e.g., subtract plasma intensity) in order to determine the feature dimension (e.g., thickness) from the reflected light intensity. 
     Physical features may be determined using multiple methods from the collected spectrum. For example, physical features may be determined by referencing a library to match the detected spectrum with a pre-calculated and pre-stored spectrum. In one implementation, direct physical regression models may be used to obtain film thickness for un-patterned wafers. Regression model may also be used to measure critical dimensions (CDs) and other pattern parameters, of simple patterns such as 2D lines. 
     In some implementations, machine learning techniques (e.g., neural network, information fuzzy network) may be used. A supervised training method trains a machine leaning algorithm to build a relationship between properties (e.g., CDs, thicknesses, etc.) of the sample and the collected spectrum. During the training phase of the machine learning method, the spectra from samples are collected. The properties associated with each sample may be obtained from CD metrology tools. Then, the machine learning algorithm is trained by using the collected spectral data and the properties of each sample. 
     At the real-time application stage, the trained machine learning algorithm is deployed to predict target end-point based on target properties of each wafer. Spectra collected during the etching process are compared with the predicted target end-point spectra to indicating the reaching of the target properties for each wafer. 
       FIG. 5  is a flowchart that shows a method  500  for in-situ monitoring of an etch process according to one example. At step  502 , the etching process recipe starts. After a certain time (e.g., Time A≥0 sec) of plasma etching at step  504 , the method  500  proceeds to step  506 . At step  506 , the spectral intensity of the reflected light beam from the substrate  116  and the spectral intensity of the background light are measured. The controller  114  processes the measured spectral intensity of the reflected light beam  122  to optionally suppress (e.g., subtract or filter) the background light to obtain background corrected spectrum during plasma etching. For example, the controller  114  can subtract the spectral intensity of the background light from the spectral intensity of the reflected light beam  122 . 
     At step  508 , a prediction algorithm, such as a machine learning algorithm or a polynomial algorithm, analyzes the acquired spectra based on a training model  514  and associates a particular property (e.g. thickness) of the substrate or structures formed thereupon to that spectrum. 
     Then, at step  510 , in response to determining that the property of the substrate or structures formed thereupon has been achieved, the process proceeds to step  512 . In response to determining that the property of the substrate or structures formed thereupon has not been achieved, the process goes back to step  506 . At step  512 , the controller  114  may modify the etching process, for example, switch or stop the recipe, or change the recipe to a different recipe when the measurement indicates that the process is getting close to completion. 
     The algorithms can also use periodic measurements on one or more reference substrates (calibration), such as a bare silicon wafer and/or thin-film wafers, to compensate for optical sensor or etch chamber components drifts. During calibration of the system, a beam may be reflected from a bare (i.e., unpatterned) silicon wafer or other wafer of known properties. The reflected beam is used to calibrate for any changes in the optical sensor  101 , for example due to the clouding of windows (e.g., windows  202  and  204 ) by products of the plasma process. The recalibration may be applied when a predetermined number of wafers have been processed in the plasma processing system  100 . 
       FIG. 6  is an exemplary schematic that shows exemplary results. Detection of thickness by the optical sensor  101  disclosed herein was compared to other detection methods and models. For example, a reference wafer map having M sites may be used. N sites out of the M sites that represent the range of a layer thickness in the wafer map are selected by the inventors. The selected N sites are indicated by circles in schematic  600 . The linear nature of the plot shown in schematic  600  indicates a good agreement between measurements made with the optical sensor  101  described herein (vertical axis) and measurements made with another tool (e.g., a metrology tool). 
     Next, a hardware description of the controller  114  according to exemplary embodiments is described with reference to  FIG. 7 . In  FIG. 7 , the controller  114  includes a CPU  700  which performs the processes described herein. The process data and instructions may be stored in memory  702 . These processes and instructions may also be stored on a storage medium disk  704  such as a hard drive (HDD) or portable storage medium or may be stored remotely. Further, the claimed advancements are not limited by the form of the computer-readable media on which the instructions of the inventive process are stored. For example, the instructions may be stored on CDs, DVDs, in FLASH memory, RAM, ROM, PROM, EPROM, EEPROM, hard disk or any other information processing device with which the controller  114  communicates, such as a server or computer. 
     Further, the claimed advancements may be provided as a utility application, background daemon, or component of an operating system, or combination thereof, executing in conjunction with CPU  700  and an operating system such as Microsoft® Windows®, UNIX®, Oracle® Solaris, LINUX®, Apple macOS™ and other systems known to those skilled in the art. 
     In order to achieve the controller  114 , the hardware elements may be realized by various circuitry elements, known to those skilled in the art. For example, CPU  700  may be a Xenon or Core processor from Intel of America or an Opteron processor from AMD of America, or may be other processor types that would be recognized by one of ordinary skill in the art. Alternatively, the CPU  700  may be implemented on an FPGA, ASIC, PLD or using discrete logic circuits, as one of ordinary skill in the art would recognize. Further, CPU  700  may be implemented as multiple processors cooperatively working in parallel to perform the instructions of the inventive processes described above. 
     The controller  114  in  FIG. 7  also includes a network controller  706 , such as an Intel Ethernet PRO network interface card from Intel Corporation of America, for interfacing with network  728 . As can be appreciated, the network  728  can be a public network, such as the Internet, or a private network such as LAN or WAN network, or any combination thereof and can also include PSTN or ISDN sub-networks. The network  728  can also be wired, such as an Ethernet network, or can be wireless such as a cellular network including EDGE, 3G, 4G and 5G wireless cellular systems. The wireless network can also be WiFi®, Bluetooth®, or any other wireless form of communication that is known. 
     The controller  114  further includes a display controller  708 , such as a NVIDIA® GeForce® GTX or Quadro® graphics adaptor from NVIDIA Corporation of America for interfacing with display  710 , such as a Hewlett Packard® HPL2445w LCD monitor. A general purpose I/O interface  712  interfaces with a keyboard and/or mouse  714  as well as an optional touch screen panel  716  on or separate from display  710 . General purpose I/O interface also connects to a variety of peripherals  718  including printers and scanners, such as an OfficeJet® or DeskJet® from Hewlett Packard. 
     A sound controller  720  is also provided in the controller  114 , such as Sound Blaster® X-Fi Titanium® from Creative, to interface with speakers/microphone  722  thereby providing sounds and/or music. 
     The general purpose storage controller  724  connects the storage medium disk  704  with communication bus  726 , which may be an ISA, EISA, VESA, PCI, or similar, for interconnecting all of the components of the controller  114 . A description of the general features and functionality of the display  710 , keyboard and/or mouse  714 , as well as the display controller  708 , general purpose storage controller  724 , network controller  706 , sound controller  720 , and general purpose I/O interface  712  is omitted herein for brevity as these features are known. 
     A system which includes the features in the foregoing description provides numerous advantages to users. The disclosed normal-incidence in-situ process monitor sensor provides increased sensitivity (signal to noise ratio) to related technologies because the normal incidence reflectometer with zero degree angle of incidence (AOI) has better measurement sensitivity. In addition, the disclosed sensor has a lower cost because only one optics module is required. The disclosed sensor has a compact design, and requires minimum chamber modification and minimum on-chamber alignment. Further, the disclosed sensor can eliminate the shutter for plasma background correction due to the increased sensitivity. For example, collection of p-polarized light reflected from the substrate  116  results in better signal purity. The disclosed sensor can be used for different wafer structures. 
     Obviously, numerous modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting of the scope of the invention, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.