Patent Publication Number: US-11662317-B2

Title: Metrology for OLED manufacturing using photoluminescence spectroscopy

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/258,392 filed Jan. 25, 2019, which claims benefit of Indian Provisional Patent Application No. 201841014177, filed Apr. 13, 2018. Each of the above-referenced applications is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to non-destructive in-situ metrology for monitoring uniformity and/or dopant concentration in a layer using static and nanosecond transient photoluminescence spectroscopy. 
     Description of the Related Art 
     Organic light-emitting diodes (OLEDs) are light-emitting diodes (LEDs) that include an organic semiconductor layer that emits light in response to an electric current. This organic semiconductor layer, referred to as an emissive electroluminescent layer, is positioned between two electrodes, one of which is typically transparent (or both electrodes in the case of transparent displays). OLEDs are used to create emissive digital displays in devices such as television screens, computer monitors, mobile phones, hand-held game consoles, and personal digital assistants (PDAs). The pixels of an OLED display are formed from the organic semiconductor layers, and therefore emit visible light themselves. As a result, unlike a liquid crystal display (LCD), an OLED display operates without a backlight. Consequently, OLED displays are generally thinner and lighter than equivalent liquid crystal displays (LCDs), and produce deeper blacks and achieve higher contrast ratios than LCDs. 
     An active-matrix OLED (AMOLED) display includes a high density array of organic electroluminescent pixels situated on a backplane that directly accesses and switches each individual pixel on or off. The electroluminescent pixels are each formed from a stack of various organic layers that are selectively deposited on the TFT backplane and bound by thin-film cathode and anode layers. The organic layers that make up each electroluminescent pixel generally include an electron injection layer (EIL), an electron transport layer (ETL), an emissive layer (EML), a hole transport layer (HTL), and a hole injection layer (HIL). The quality and uniformity of each of these layers can significantly affect the performance of the pixel and the OLED display as a whole. For example, a variation in dopant concentration in a layer as small as a fraction of 1% can alter the dynamics of charge carriers in the layer, which in turn affects the photoluminescent behavior of the layer. Variations in thickness of one or more of these organic layers can also impact device efficiency. 
     The various organic layers of an OLED are typically formed in a single high-vacuum deposition system, where each layer is deposited on the backplane via a different chamber of the system. As a result, an OLED device cannot be accessed and tested until the entire OLED formation process has completed, which can last up to several hours, during which a large number of substrates are typically processed. Consequently, a process excursion in a single chamber can affect a large number of devices before being detected. Thus, such a delay in the detection of and response to a process excursion can be costly in terms of yield loss. 
     In addition, in the current state of the art, metrology techniques for OLED devices are relatively time-consuming and are employed after all organic layers have been deposited, which can also delay detection of process excursions. Further, conventional metrology techniques for OLED devices are generally most accurate when applied to a significantly thicker OLED layer than actual OLED device layer thickness. As a result, the signal produced when measuring the small changes associated with production layers of OLED devices can be inadequate for generating reliable feedback for the deposition process. 
     Accordingly, there is a need in the art for systems and methods that enable fast and accurate monitoring of the properties of the individual layers of an OLED device. 
     SUMMARY 
     According to various embodiments, an apparatus comprises: a light source that generates an excitation light that includes light from the visible or near-visible spectrum; an optical assembly configured to direct the excitation light onto a photoluminescent (PL) layer formed on a substrate that is disposed in a system for depositing the PL layer; a detector that is configured to receive a PL emission generated by the PL layer in response to the excitation light interacting with the PL layer and generate a signal based on the PL emission; and a computing device coupled to the detector and configured to receive the signal from the detector and determine a characteristic of the PL layer based on the signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments. 
         FIG.  1 A  is a schematic illustration of the various layers of a typical organic light-emitting diode (OLED) device. 
         FIG.  1 B  is a schematic illustration of an OLED fabrication system in which various embodiments of the present disclosure can be implemented. 
         FIG.  2    is a conceptual block diagram of an OLED layer monitoring system, according to various embodiments of the present disclosure. 
         FIG.  3 A  is an energy diagram illustrating the emission of fluorescent light from a photoluminescent (PL) material, according to various embodiments of the present disclosure 
         FIG.  3 B  is an energy diagram illustrating the emission of phosphorescent light from a PL material, according to various embodiments of the present disclosure. 
         FIG.  4    is a schematic diagram illustrating an OLED monitoring system, configured according to various embodiments of the present disclosure. 
         FIG.  5 A  is a graph illustrating multiple PL intensity spectra generated via a static photoluminescence measurement assembly that demonstrate PL peak intensity variation with respect to dopant concentration, according to an embodiment of the present disclosure. 
         FIG.  5 B  is a graph illustrating multiple PL intensity spectra generated via a static photoluminescence measurement assembly that demonstrate PL peak intensity variation with respect to PL thickness, according to an embodiment of the present disclosure. 
         FIG.  6    is a graph illustrating a first PL intensity decay curve and a second PL intensity decay curve generated via a transient photoluminescence measurement assembly that demonstrate variation of PL intensity decay as a function of dopant concentration, according to an embodiment of the present disclosure. 
         FIG.  7    is a schematic illustration of an OLED monitoring system, configured according to various embodiments of the present disclosure. 
         FIG.  8    is a schematic illustration of a fiber-based OLED monitoring system, configured according to various embodiments of the present disclosure. 
         FIG.  9    is a schematic cross-sectional view of a probe of an array in the fiber-based OLED monitoring system of  FIG.  8   , according to various embodiments of the present disclosure. 
         FIG.  10    is a flow chart of process steps for determining a film characteristic of the PL material, according to various embodiments of the disclosure. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a more thorough understanding of the embodiments of the present disclosure. However, it will be apparent to one of skill in the art that one or more of the embodiments of the present disclosure may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring one or more of the embodiments of the present disclosure. 
       FIG.  1 A  is a schematic illustration of the various layers of a typical organic light-emitting diode (OLED) device  100 . As shown, OLED device  100  includes a plurality of organic semiconductor layers  120  formed between a cathode layer  101  and an anode layer  102 . Cathode layer  101 , anode layer  102 , and organic semiconductor layers  120  are disposed on a glass or other substrate  103 , and can each be formed via the selective deposition of thin films using thin film deposition techniques known in the art. In one example, OLED device  100  is a single pixel in an OLED display. 
     Organic semiconductor layers  120  can include, without limitation, an electron transport layer (ETL)  121 , a hole-blocking layer (HBL)  122 , an emissive layer (EML)  123 , a hole transport layer (HTL)  124 , and a hole injection layer (HIL)  125 , among others. Together, organic semiconductor layers  120  provide the light-emitting functionality of OLED device  100 . In  FIG.  1   , OLED device  100  is depicted with five organic semiconductor layers, but in some cases OLED device  100  includes more or fewer organic semiconductor layers. For example, organic semiconductor layers  120  may include, additionally or alternatively, an electron injection layer, an electron blocking layer, and the like. 
     For proper operation of OLED device  100 , such as uniform color and brightness relative to other pixels in the same OLED display, certain film characteristics for each of the organic semiconductor layers  120  should be maintained within a specified range. Examples of such characteristics include thickness and thickness uniformity of a layer, the concentration of dopant molecules within the host molecules of a layer (dopant concentration), and uniformity of the dopant concentration across a layer (dopant uniformity). According to various embodiments of the present disclosure, some or all of these film characteristics can be measured non-destructively after being deposited on substrate  103  while substrate  103  is in-situ, in-line, and/or at end-of-line, as illustrated in  FIG.  1 B . 
       FIG.  1 B  is a schematic illustration of an OLED fabrication system  150  in which various embodiments of the present disclosure can be implemented. As shown, OLED fabrication system  150  includes multiple deposition chambers  150 - 1 ,  150 - 2 , . . .  150 N, each configured to deposit one of N different OLED layers on a substrate, such substrate  103  in  FIG.  1 A . In addition, OLED fabrication system  150  includes transfer chambers  151  disposed between each of deposition chambers  150 - 1 - 150 -N and a load lock  152  for the removal of substrates  103 . Generally, prior to removal of substrates  103  and after all active layers are completed, a so-called the capping layer (CPL), which is not shown in  FIG.  1 A , is deposited on substrate  103 . CPLs are typically transparent organic materials that are similar in composition to other OLED materials. 
     According to various embodiments of the present disclosure, certain OLED characteristics can be measured non-destructively after being deposited on substrate  103  while substrate  103  is still in-situ, i.e., while substrate  103  is within the deposition chamber that is forming a particular organic semiconductor layer  120 . Alternatively or additionally, in some embodiments, certain film characteristics can be measured non-destructively immediately after being deposited on substrate  103 , when substrate  103  is in-line. That is, the film characteristics are measured when substrate  103  is disposed between two of the deposition chambers of OLED fabrication system  150 . For example, the in-line measurement(s) can be performed when substrate  103  is disposed in a transfer chamber  151 . Thus, in such embodiments, the film characteristics for a specific semiconductor layer  120  is measured non-destructively in real-time, after being deposited on substrate  103  and before the deposition of subsequent semiconductor layers  120 . Alternatively or additionally, in some embodiments, certain film characteristics can be measured non-destructively when substrate  103  is at end-of-line, i.e., when deposition processes have been completed on substrate  103 , but substrate  103  has not been removed from OLED fabrication system  150 . That is, the film characteristics are measured while substrate  103  is disposed in load lock  152  or some other end-of-line chamber of OLED fabrication system  150 . In such embodiments, the film characteristics can be measured prior to deposition of a thin film encapsulant layer on substrate  103  and prior to exposure of substrate  103  to atmosphere. In any of these situations, monitoring of one or multiple film characteristics is performed without stopping the fabrication process. As a result, production losses associated with system idle time are minimized or otherwise reduced. Further, because the monitoring of such film characteristics can be performed immediately after deposition of each organic semiconductor layer  120 , process issues with a specific deposition chamber can be detected in real time, and not after a complete batch of substrates  103  has completed processing. 
       FIG.  2    is a conceptual block diagram of an OLED layer monitoring system  200 , according to various embodiments of the present disclosure. OLED layer monitoring system  200  is configured to provide in-situ and non-destructive monitoring of dopant concentration and film thickness of some or all of the organic semiconductor layers  120  shown in  FIG.  1   . In addition, the measurements performed by OLED layer monitoring system  200  can provide dopant concentration level and uniformity and film thickness and its uniformity for one or more of organic semiconductor layers  120 . OLED layer monitoring system  200  includes a light source  220  configured to generate an excitation light  201 , a detector  230  configured to receive a photoluminescent (PL) emission  202 , and a computing device  250  coupled to detector  230 . OLED layer monitoring system  200  further includes an optical assembly  240  configured to direct excitation light  201  to a PL layer  205  formed on a substrate  203  and to direct PL emission  202  to detector  230 . 
     As shown, OLED layer monitoring system  200  performs one or more measurements on PL layer  205  while substrate  203  is disposed within a deposition system  290 . Specifically, OLED layer monitoring system  200  causes PL layer  205  to be excited with excitation light  201  and measures PL emission  202  that occurs as a result of the excitation of PL layer  205 . Computing device  250  then determines one or more film characteristics of PL layer  205  based on the measured excitation, as described below. Deposition system  290  can be any technically feasible system for depositing one or more PL layers  205  on substrate  203 . For example, in some embodiments, deposition system  290  includes one or more evacuated deposition chambers that have a low partial pressure of oxygen therein during processing. Alternatively or additionally, deposition system  290  includes one or more atmospheric pressure or low-vacuum deposition chambers that can operate with an inert gas disposed therein. 
     PL layer  205  can be an OLED layer or any other layer of material that emits light of a first frequency in response to excitation from light of a second frequency that is higher than the first frequency. For example, PL layer  205  can include a layer of material that includes quantum dots, light-emitting diodes and the like. In some embodiments, PL layer  205  includes an organic photo-luminescent layer. 
     Light source  220  can be any technically feasible light source that generates a suitable excitation light  201  for exciting PL layer  205 . In some embodiments, light source  220  is configured to generate excitation light  201  is selected in wavelength and intensity so that the chemical properties of PL layer  205  are not chemically altered. For example, in some embodiments, the power of excitation light  201  can be limited to avoid photo-bleaching of PL layer  205 . In one such embodiment, the power of excitation light  201  can be limited to no more than about 1 μW. In other embodiments, the power of excitation light  201  can be varied depending on the particular material included in PL layer  205  and the duration of exposure of PL layer  205  to excitation light  201 . Thus, in some embodiments, the power of excitation light  201  can as much as about 10 μW. 
     In some embodiments, light source  220  includes one or more lasers that each generate a specific excitation light  201  for a particular PL layer  205 , so that the particular PL layer  205  can be excited by the appropriate specific excitation light. For example, in such embodiments, light source  220  may include a tunable laser that selectively generates a first wavelength light (e.g., 405 nm light) for exciting a first PL layer  205  and a second wavelength light (e.g., 375 nm light) for exciting a second PL layer  205 . Alternatively or additionally, in such embodiments, light source  220  may include multiple lasers that are each employed for generating excitation light  201  for a different PL layer  205 . In some embodiments, light source  220  includes a broadband light source, such as a white plasma-based light source, a white light-emitting diode (LED), or some other light source that generates light in the 350-400 nm wavelength range. In some embodiments, light source  220  is a single light source that is employed to generate excitation light  201  having the same frequency for the excitation of any PL layer  205  that is measured by OLED layer monitoring system  200 . For example, in such an embodiment, light source  220  includes a laser that generates an excitation light  201  having a single fixed frequency of light or a single fixed range of frequencies of light between about 300 nm and about 450 nm. 
     For clarity, in the embodiment illustrated in  FIG.  2   , excitation light  201  is depicted to be incident on PL layer  205  at a non-normal angle of incidence  209 . In other embodiments, angle of incidence  209  can be 90°, or any other suitable angle for a particular configuration of OLED layer monitoring system  200 , 
     Detector  230  is configured to receive PL emission  202  when PL layer  205  is excited by excitation light  201 , and can include any suitable light detector. As will be discussed further below, the PL emission  202  will have a different set of wavelengths from the one or more wavelengths found in excitation light  201 . For example, in some embodiments computing device  250  employs a spectral intensity of PL emission  202  to determine one or more film characteristics of PL layer  205 . In such embodiments, detector  230  includes a spectrometer configured to quantify a radiant intensity for each of a plurality of wavelengths of light included in PL emission  202 . In such embodiments, the spectrometer generally includes a grating and/or other optical elements to spatially disperse the various frequencies of light included in PL emission  202 . In addition, a suitable detector is optically coupled to or included in the spectrometer, such as an array of photodetectors or charge-coupled devices (CCDs) that each quantify a PL intensity for a different portion of the spectrum of PL emission  202 . Thus, the spectrally dispersed PL light is imaged by the CCD image sensor pixels at the focal plane of the spectrometer where the CCD image sensor is located. The CCD pixels are calibrated for the wavelength range with a suitable calibration lamp such that each pixel represents a specific wavelength and the PL spectrum can be directly recorded on the CCD sensor. 
     Alternatively or additionally, in some embodiments, computing device  250  employs a single intensity value associated with PL emission  202  to determine one or more film characteristics of PL layer  205 . In such embodiments, detector  230  includes a suitable device for quantifying incident light intensity, such as a photomultiplier tube or photon-counters. In such embodiments, detector  230  may further include an optical filter or other optical element configured to selectively transmit light of a specified wavelength or wavelength band, so that light detected by detector  230  is limited to the specified wavelength or wavelength band. 
     Computing device  250  includes logic configured to receive signals from detector  230  and to determine one or more film characteristics of one or multiple PL layers  205  formed on substrate  203 . Computing device  250  can be any computing device suitable for practicing one or more embodiments of the present disclosure. Computing device  250  may be implemented as a central processing unit (CPU), a graphics processing unit (GPU), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), any other type of processing unit, or a combination of different processing units, such as a CPU configured to operate in conjunction with a GPU. In general, computing device  250  may be any technically feasible hardware unit capable of processing data and/or executing software applications for implementing one or more embodiments of the present disclosure. Further, computing device  250  may correspond to a physical computing system, or may be a virtual computing instance executing within a computing cloud. In some embodiments, the functionality of computing device  250  is incorporated into deposition system  290 . 
     In some embodiments, computing device  250  determines a dopant concentration of PL layer  205  by measuring a transient photoluminescence of PL layer  205  and comparing the measured transient photoluminescence to a previously established calibration curve, table, or function. As described below, transient photoluminescence in organic semiconductor layers is a function of dopant concentration, and is generally unaffected by thickness of the organic semiconductor layer. Transient photoluminescence (TPL) of an organic semiconductor layer is generally determined by measuring the decay over time of the photoluminescence intensity, at one or more wavelengths, generated by exposing a portion of an organic semiconductor layer to an amount of radiation generated by a light source (e.g., pulse of the excitation light  201 ). In TPL, the photoluminescence spectrum is monitored as a function of delay between excitation pulse and CCD gate pulse that records the PL intensity. The kinetics of PL decay is indicative of the outflow of excitation energy, which in turn is indicative of the host:dopant ratio in the emissive layer. In the emissive layer, the host molecular system is excited, which in turn transfers the excitation energy via intersystem crossing over to the excited dopant molecular system. Subsequently, the excited dopant relaxes to the ground state, releasing energy in form of photoluminescence. Hence, the larger the dopant concentration, the faster the PL decay, which is what is observed in the TPL monitoring system described herein. The PL decay kinetics are independent of layer thickness but very sensitive to the host:dopant ratio in the emissive layer. 
     In some embodiments, computing device  250  determines a doping concentration of PL layer  205  by performing a thickness measurement (for example via reflectometry) and measuring a static photoluminescence of PL layer  205 . Based on the thickness measurement and the static photoluminescence measurement, computing device  250  then determines the dopant concentration. Specifically, computing device  250  can determine the dopant concentration of PL layer  205  by comparing the measured static photoluminescence to a previously established calibration curve, table, or function that is associated with the measured thickness of a particular PL layer  205 . Thus, even though the static photoluminescence of PL layer  205  is a function of both the thickness of PL layer  205  and the concentration of dopant included in PL layer  205 , computing device  250  can determine a thickness of PL layer  205  by measuring static photoluminescence of PL layer  205 . 
     Alternatively or additionally, in some embodiments, computing device  250  determines a thickness of PL layer  205  by measuring a transient photoluminescence of PL layer  205  and a static photoluminescence of PL layer  205 . In such embodiments, computing device  250  first determines a dopant concentration of PL layer  205  by measuring a transient photoluminescence of PL layer  205  and comparing the measured photoluminescence to a first calibration curve, table, or function. Computing device  250  then determines a thickness of PL layer  205  by measuring a static photoluminescence of PL layer  205  and comparing the measured static photoluminescence to a second calibration curve, table, or function for the dopant concentration of PL layer  205  that is determined based on transient photoluminescence. 
     As noted above, optical assembly  240  is configured to direct excitation light  201  to PL layer  205  and to direct PL emission  202  to detector  230 . Optical assembly  240  can include any of various configurations, depending on the which film characteristic or characteristics of PL layer  205  are determined by computing device  250 . The configuration of optical assembly can also depend on which information associated with PL emission  202  is employed by computing device  250  to determine the film characteristic or characteristics of PL layer  205 . 
     As noted above, PL emission  202  is generated by PL layer  205  when excitation light  201  is incident on PL layer  205 . More specifically, atoms within a host material of PL layer  205  and/or a dopant material in PL layer  205  contribute to the generation of PL emission  202 . The host material of PL layer  205  can include any PL material, which is a material that emits light by photoexcitation, i.e. in response to the absorption of photons. For example, the organic semiconductors included in OLED devices are typically PL materials. Examples of such host PL materials include CBP (4,4′-Bis(N-carbazolyl)-1,1′-Biphenyl), TCTA (4,4′,4″-Tris(carbazole-9-yl)triphenylamine) or TPBi (2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)). Examples of dopant materials that can be included in PL layer  305  include green emitter molecules like Ir(ppy) 2 (acac) (Bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III)) (green emitter), red emitter molecules like Ir(btpy) 3  (Tris(2-(benzo[b]thiphen-2-yl)pyridineiridium(III)) and blue emitter molecules like Bebq2 (Bis(10-hydroxybenzo[h]quinolinato)beryllium). PL emission  202  can include photons emitted via fluorescence, phosphorescence, or a combination of both, as illustrated in  FIGS.  3 A and  3 B . 
       FIG.  3 A  is an energy diagram illustrating the emission of fluorescent light  301  from a fluorescent material, and  FIG.  3 B  is an energy diagram illustrating the emission of phosphorescent light  302  from a phosphorescent material, according to various embodiments of the present disclosure. As shown in  FIG.  3 A , the emission of fluorescent light  301  (i.e., fluorescence) occurs from vibrational state v 0  of excited singlet S 1  down to S 0  (and its vibrational levels). When absorption  311  of incident light  312  that exceeds the bandgap of the PL material (typically in the visible to ultra-violet range) occurs, electrons in the PL material are excited to higher vibrational states of S 1 . These excited vibrational energy states relax via non-radiative transfer  313  to the ground vibrational state of S 1 . Eventually the molecules of the PL material relax  314  to a ground electronic state S 0 , emitting fluorescent light  301 . Usually fluorescence occurs at a wavelength that is significantly red-shifted from the excitation wavelength (Stokes shift), and is unique for every fluorescent material. Typical fluorescence lifetimes range between femtoseconds (fs) to picoseconds (ps). 
     As shown in  FIG.  3 B , the emission of phosphorescent light  302  (i.e., phosphorescence) occurs from higher triplet state (T 1 ) to singlet ground state (S 0 ) after the singlet excitation undergoes the inter-system crossing  321  from higher singlet (S 1 ) of the host PL material to the higher triplet state (T 1 ) of a dopant material within the PL material. Relaxation  322  of molecules from T 1  to S 0  emits Stokes&#39; shifted photons collectively known as phosphorescence (phosphorescent light  302 ). The decay lifetimes are very long ranging anywhere between nanoseconds (ns) to milliseconds (ms) and even hours, depending on the PL material and surrounding conditions. As employed herein, the term “photoluminescence” is a collective term for material emission processes that include fluorescence, phosphorescence, or a combination of both. Thus, PL emission  202  of  FIG.  2    can include fluorescent light  301 , phosphorescent light  302 , or a combination of both. 
     It is noted that incident light  312  generally includes a first wavelength or group of wavelengths, while fluorescent light  301  includes a second wavelength or group of wavelengths that is different from the first wavelength or group of wavelengths. Similarly, phosphorescent light  302  includes a third wavelength or group of wavelengths that is different from the first wavelength or group of wavelengths. Thus, while the energy that causes the generation of PL emission  202  originates from excitation light  201 , the light making up PL emission  202  includes different photons at different energies than the photons of excitation light  201 . 
     According to some embodiments, excitation light  201  includes light in the visible light spectrum, i.e., light of wavelengths from about 400 nm to about 700 nm. According to some embodiments, excitation light  201  includes light in the near-infrared spectrum, which includes light of wavelengths from about 700 nm to about 800 nm. According to some embodiments, excitation light  201  includes light in the mid-infrared spectrum, which includes light of wavelengths from about 800 nm to about 3000 nm (3 microns). According to some embodiments, excitation light  201  includes light in the near ultraviolet spectrum, which includes light of wavelengths from about 100 nm to about 400 nm. It is noted that X-rays are generally considered to include light of wavelengths of about 0.01 nm to about 10 nm, and which does not overlap with the wavelengths of light in the near ultraviolet spectrum. Thus, unlike analytical techniques that employ X-rays to generate fluorescent emissions, such as X-ray fluorescence (XRF), embodiments described herein employ much less energetic photons to generate PL emission  202 . In some embodiments, PL emission  202  includes light having a wavelength in the visible spectrum. Alternatively or additionally, in some embodiments, PL emission  202  includes light having a wavelength in the near infra-red spectrum and/or the mid-infrared spectrum. In some embodiments, PL emission  202  includes light having a wavelength in the near-visible spectrum, which can include light from the near infra-red spectrum to the near ultraviolet spectrum. 
     In some embodiments, an OLED monitoring system is configured to determine one or more film characteristics of a PL layer by measuring both static photoluminescence and/or transient photoluminescence of a PL material. One such embodiment is illustrated in  FIG.  4   .  FIG.  4    is a schematic diagram illustrating an OLED monitoring system  400 , configured according to various embodiments of the present disclosure. OLED layer monitoring system  400  includes a laser  401 , a laser electronic synchronization module  402 , a static photoluminescence measurement assembly  420  that acts as a first detector, and a transient photoluminescence measurement assembly  430  that acts as a second detector. In some embodiments, OLED layer monitoring system  400  further includes an optical assembly  440  that directs the excitation light  201  and PL emission  202  as shown. Optical assembly  440  can include free-space optical elements, such as a beam splitter  441  and mirror  442 , and/or fiber optic elements (not shown). OLED layer monitoring system  400  typically also includes a computing device for receiving signals from static photoluminescence measurement assembly  420  and transient photoluminescence measurement assembly  430  and for determining one or more film characteristics of PL layer  205 . For clarity, the computing device of OLED layer monitoring system  400  is omitted in  FIG.  4   . 
     Laser  401  is a laser configured to generate timed pulses on a time scale that corresponds to the PL decay period of PL layer  205  when excited by a suitable frequency of incident light. For example, in some embodiments, laser  401  is configured to generate timed pulses on the picoseconds (ps) timescale. Laser electronic synchronization module  402  synchronizes the output of laser  401  with the data collection of static photoluminescence measurement assembly  420  and transient photoluminescence measurement assembly  430 . 
     Static photoluminescence measurement assembly  420  is configured to measure steady-state photoluminescence of PL layer  205  when PL layer  205  is excited by laser  401 . In embodiments in which PL layer  205  is excited by laser  401 , laser  401  can operate either as a continuous-wave (CW) laser or pulsed laser. In some embodiments, static photoluminescence measurement assembly  420  is configured to measure spectral information associated with PL emission  202 . In such embodiments, static photoluminescence measurement assembly  420  includes a spectrometer  421  for spatially separating the frequencies of PL emission  202  and a light detector  422  for quantifying the radiant intensity for each wavelength of interest in PL emission  202 . Alternatively, spectrometer  421  can be replaced by any suitable optical element or elements that spatially disperse the various frequencies of light included in PL emission  202 . In some embodiments, light detector  422  includes an array of light detectors, such as a CCD array or CMOS array, where each light detector in the array measures a radiant intensity for a particular wavelength or wavelength band of interest. Thus, in operation, static photoluminescence measurement assembly  420  generates an intensity spectrum of PL emission  202  that facilitates measurement of one or more film characteristics of PL layer  205 . Examples of such PL intensity spectra are illustrated in  FIGS.  5 A and  5 B . 
       FIG.  5 A  is a graph  500  illustrating multiple PL intensity spectra  501 - 503  generated via static photoluminescence measurement assembly  420  that demonstrate PL peak intensity variation with respect to dopant concentration, according to an embodiment of the present disclosure. Each of PL intensity spectra  501 - 503  is generated for a different 50 nm thick PL layer  205  of Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) that contains the dopant Ir(ppy) 2 (acac). Specifically, PL intensity spectrum  501  is generated for a TCTA layer that includes 1% Ir(ppy) 2 (acac); PL intensity spectrum  502  is generated for a TCTA layer that includes 3% Ir(ppy) 2  (acac); and PL intensity spectrum  503  is generated for a TCTA layer that includes 7% Ir(ppy) 2 (acac). As shown, the magnitude of peak intensities  501 A,  502 A, and  503 A varies as a function of dopant concentration, where increasing dopant concentration in the TCTA layer results in an increase in the magnitude of peak intensity. Specifically, in the embodiment illustrated in  FIG.  5 A , the magnitude of peak intensities  501 A,  502 A, and  503 A are 186279±2.7%, 198084±2.9%, and 211718±1.9%, respectively. In addition, the peak wavelength of PL intensity spectra  501 - 503  increases (i.e., red-shifts) with increasing dopant concentration. Specifically, in the embodiment illustrated in  FIG.  5 A , the peak wavelengths of PL intensity spectra  501 - 503  are 517.89 nm, 518.20 nm, and 520.69 nm, respectively. 
     Thus, in some embodiments, for a known thickness of PL layer  205  of a particular PL material, a PL intensity spectrum generated by static photoluminescence measurement assembly  420  can indicate the concentration of dopant included in PL layer  205 . In such embodiments, a computing device of OLED layer monitoring system  400  compares a PL intensity spectrum for the PL layer  205  of known thickness to previously established calibration curves for that thickness of PL layer  205  and determines the concentration of dopant in the PL layer  205  of known thickness. Alternatively or additionally, the computing device of OLED layer monitoring system  400  can determine a specific value from the PL intensity spectrum for the PL layer  205  of known thickness, such as a magnitude of the peak intensity of the PL intensity spectrum and/or a wavelength of the peak intensity of the PL intensity spectrum. The computing device then compares the specific value or values to a previously established calibration table or function to determine the concentration of dopant in the PL layer  205  of a known thickness. It is noted that for each particular thickness and particular PL material, a different calibration process is typically employed for determining the concentration of dopant included in PL layer  205 . 
     Conversely, when PL layer  205  includes a known concentration of dopant, a PL intensity spectrum generated by static photoluminescence measurement assembly  420  enables the computing device of OLED layer monitoring system  400  to determine the thickness of PL layer  205  using calibration curves, tables, or functions in a similar fashion. For example,  FIG.  5 B  is a graph  550  illustrating multiple PL intensity spectra  551 - 553  generated via static photoluminescence measurement assembly  420  that demonstrate PL peak intensity variation with respect to PL thickness, according to an embodiment of the present disclosure. Each of PL intensity spectra  551 - 553  is generated for a respective PL layer  205  of a different thickness of Tris(8-hydroxyquinoline)aluminum(III), commonly known as AlQ3. Specifically, PL intensity spectrum  551  is generated for a first AlQ3 layer that is 10 nm thick, PL intensity spectrum  552  is generated for a second AlQ3 layer that is 30 nm thick, and PL intensity spectrum  553  is generated for a third AlQ3 layer that is 50 nm thick, where the first, second, and third layers each have the same dopant concentration. As shown, the magnitude of the peak intensities of PL intensity spectra  551 - 553  varies significantly as a function of dopant concentration. 
     Returning to  FIG.  4   , transient photoluminescence measurement assembly  430  is configured to measure transient photoluminescence of PL layer  205  when PL layer  205  is excited by laser  401 . In some embodiments, transient photoluminescence measurement assembly  430  is configured to measure PL intensity information associated with PL emission  202  via time-correlated single photon counting (TCSPC). In TCSPC, the time-dependent intensity profile of PL emission  202  is recorded in the time domain when PL emission  202  occurs upon excitation by a short flash of light, such as a laser pulse from laser  401 . In such embodiments, transient photoluminescence measurement assembly  430  includes a light detector  431  that is configured for the precisely timed registration of single photons of PL emission  202 . For example, in some embodiments, light detector  431  includes a single-photon sensitive detector, such as a photomultiplier tube (PMT), a micro channel plate (MCP), a single photon avalanche diode (SPAD), or a hybrid PMT. For sufficient sensitivity in measuring the time decay of PL emission  202 , in some embodiments, the measurements of PL emission  202  by light detector  431  are based on precisely times repetitive excitations of PL layer  205 . The reference for the timing of the excitations and associated measurements can be the corresponding excitation pulse, which is provided by laser electronic synchronization module  402 . In operation, transient photoluminescence measurement assembly  430  generates a PL intensity decay curve that facilitates measurement of one or more film characteristics of PL layer  205 . Examples of such PL intensity decay curves are illustrated in  FIG.  6   . 
       FIG.  6    is a graph  600  illustrating a first PL intensity decay curve  601  and a second PL intensity decay curve  602  generated via transient photoluminescence measurement assembly  430  that demonstrate variation of PL intensity decay as a function of dopant concentration, according to an embodiment of the present disclosure. First PL intensity decay curve  601  and second PL intensity decay curve  602  are each generated for a different 50 nm thick PL layer  205  of TCTA that contains the dopant (ppy) 2 Ir(acac). Specifically, first PL intensity decay curve  601  is generated for a PL layer  205  that includes 5% (ppy) 2 Ir(acac) and second PL intensity decay curve  602  is generated for a PL layer  205  that includes 7% (ppy) 2 Ir(acac). The PL intensity counts (Y-axis of graph  600 ) are shown in arbitrary units, and depict the discrete intensities of light, such as counts, measured over time after PL layer  205  is excited by excitation light  201 . 
     In contrast to the PL intensity spectra  501 - 503  of  FIG.  5   , in the embodiment illustrated in  FIG.  6   , each data point is based on a number of photons measured by transient photoluminescence measurement assembly  430  over a single band of wavelengths, such as 520-530 nm (green emitters) or 620-630 nm (red emitters). That is, spectral dispersion of PL emission  202  is not performed, and photons within a predetermined range of wavelengths of PL emission  202  are measured. To that end, in some embodiments, transient photoluminescence measurement assembly  430  also includes an optical filter  432  that is configured to limit the wavelengths of PL emission  202  received by transient photoluminescence measurement assembly  430  to the predetermined range of wavelengths. In general, the predetermined range of wavelengths that is sampled by transient photoluminescence measurement assembly  430  is a relatively wide band compared to each of the wavelength bands associated with each data point in the PL intensity spectra  501 - 503  of  FIG.  5   . Furthermore, the predetermined range of wavelengths sampled by transient photoluminescence measurement assembly  430  can be selected based on the host material and/or dopant material of PL layer  205 . 
     As illustrated by first PL intensity decay curve  601  and second PL intensity decay curve  602 , the decay over time of the intensity of PL emission  202  after a discrete excitation by incident light varies as a function of dopant concentration, but not thickness of PL layer  205 . That is, increasing dopant concentration in PL layer  205  results in an increase in the rate of decay of PL emission  202 , while a change in thickness of PL layer  205  has no significant effect on the rate of decay of PL emission  202 . As a result, in some embodiments, a PL intensity decay curve generated by transient photoluminescence measurement assembly  430  can indicate the concentration of dopant included in PL layer  205 . In such embodiments, PL intensity values are collected at different times after a triggering laser pulse from laser  401  excites a PL layer  205  that includes a particular host material and dopant material. The computing device of OLED layer monitoring system  400  then constructs a PL intensity curve similar to first PL intensity decay curve  601  or second PL intensity decay curve  602 , and compares the constructed PL intensity decay curve to previously established calibration curves for a similar PL layer  205 . Based on the comparison, the computing device determines dopant concentration in PL layer  205 . In such embodiments, any suitable curve-fitting algorithm can be employed by the computing device to determine dopant concentration in PL layer  205 . 
     In one such embodiment, the computing device of OLED layer monitoring system  400  can determine a value for a fitting parameter for the PL intensity decay curve constructed for a PL layer  205 . The computing device then determines a dopant concentration of PL layer  205  based on that specific value of the fitting parameter. For example, the computing device can determine a dopant concentration by comparing the specific value of the fitting parameter to a calibration table of previously established values for the fitting parameter that is generated using PL layers having a known dopant concentration. In such an embodiment, a double-exponent fitting equation, such as Equation 1, can be employed to determine a PL intensity decay curve for a known dopant concentration that most closely matches the PL intensity decay curve constructed for PL layer  205 . 
     
       
         
           
             
               
                 
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     In equation 1, Y0 is a y-axis (PL count) offset, XO is a x-axis (time delay) constant, and A 1 , A 2 , τ 1 , and τ 2  are additional fitting parameters. In embodiments in which Equation 1 is employed to determine a dopant concentration of PL layer  205 , τ 2  can be selected as the fitting parameter that is compared to known calibration values. For example, in the embodiment illustrated in  FIG.  6   , for intensity decay curve  601 , A 1 =2137, A 2 =2568, τ 1 =220 ps and τ 2 =1.04 ns, whereas for intensity decay curve  602  A 1 =3693, A 2 =1506, τ 1 =135.6 ps, and τ 2 =1.15 ns 
     In some embodiments, an OLED monitoring system includes free-space optical elements for directing excitation light from a light source to a PL layer, and for directing a PL emission from the PL layer to one or more detectors. One such embodiment is illustrated in  FIG.  7   .  FIG.  7    is a schematic illustration of an OLED monitoring system  700 , configured according to various embodiments of the present disclosure. OLED monitoring system  700  includes one or more free-space optical elements that interact with and/or direct excitation light  201 , PL emission  202 , or both excitation light  201  and PL emission  202 . 
     For example, in some embodiments, OLED monitoring system  700  includes one or more filters  701 , such as a neutral-density filter or other optical filter for modifying the intensity or color distribution of excitation light  201 . In some embodiments, OLED monitoring system  700  includes one or more lenses  702  to shape and/or focus excitation light  201 . In some embodiments, OLED monitoring system  700  includes a dichroic mirror  703  that is highly reflective for wavelengths associated with excitation light  201  (e.g., on the order of about 400 nm) and highly transmissive for wavelengths associated with PL emission  202  (e.g., on the order of about 600 nm). In some embodiments, OLED monitoring system  700  includes an objective lens  704  configured to focus excitation light  201  onto a measuring location  720  on PL layer  205 . In such embodiments, objective lens  704  may also be configured to focus PL emission  202  onto static PL measurement assembly  420  and/or transient PL measurement assembly  430 . In some embodiments, OLED monitoring system  700  includes a confocal pinhole  705  that is configured to block out-of-focus excitation light  201  and is positioned between objective lens  704  and laser  401 . In some embodiments, OLED monitoring system  700  includes a filter  706 , such as a notch filter, configured to stop unwanted frequencies of light from reaching static PL measurement assembly  420  and/or transient PL measurement assembly  430 , such as frequencies associated with excitation light  201 . In some embodiments, OLED monitoring system  700  includes a beam-splitter  707  configured to direct a portion of PL emission  202  to static PL measurement assembly  420  and a portion of PL emission  202  to transient PL measurement assembly  430 . In some embodiments, OLED monitoring system  700  includes static PL measurement assembly  420  and/or transient PL measurement assembly  430 . 
     In some embodiments, OLED monitoring system  700  includes additional optical elements or fewer optical elements than those depicted in  FIG.  7   . Further, in some embodiments, one or more of the free-space optical elements depicted in  FIG.  7    can be replaced with one or more fiber-based components of substantially equivalent functionality. 
     In some embodiments, OLED monitoring system  700  controls or is communicatively connected to a movable stage  710  for translating substrate  203  relative to objective lens  704 . In such embodiments, movable stage  710  is configured to translate substrate  203  in a direction perpendicular to the direction of incident excitation light  201 . In such embodiments, stage  710  is configured to translate horizontally, i.e., in the direction indicated by arrow  711 , or vertically, i.e., out of the page in  FIG.  7   . In some embodiments, movable stage  710  is an X-Y stage configured to translate substrate  203  both horizontally and vertically, so that excitation light  201  can be directed to a plurality of measurement locations  720  that are distributed in two dimensions on PL layer  205 . Alternatively or additionally, in some embodiments, movable state  710  is further configured with additional motion capability, such as Z-motion (which is perpendicular to X- and Y-motion) and rotational motion, for example to offset correction of focal length sensitivity. 
     In some embodiments, movable stage  710  is disposed within the process chamber that has deposited PL layer  205 . In one such embodiment, movable stage  710  is disposed within the deposition chamber that has deposited PL layer  205 , and measurements of PL layer  205  are performed before substrate  203  is removed from the deposition chamber. In another embodiment, movable stage  710  is disposed within the deposition system but outside the deposition chamber that has deposited PL layer  205 . For example, movable stage  710  can be disposed within a transfer chamber of the deposition system, and measurements of PL layer  205  are performed after substrate  203  is removed from the deposition chamber that has deposited PL layer  205 . 
     In the embodiment illustrated in  FIG.  7   , OLED monitoring system  700  directs excitation light  201  to a single measuring location  720  on PL layer  205 . In other embodiments, when substrate  203  is in a particular position relative to OLED monitoring system  700 , OLED monitoring system  700  is configured to direct excitation light  201  to multiple measuring locations  720  on PL layer  205 . For example, in one such embodiment, OLED monitoring system  700  includes a linear array of N objective lenses  704  that extends out of the page. Thus, OLED monitoring system  700  is configured to direct excitation light  201  to N locations on PL layer  205  without repositioning substrate  203 . As a result, PL emission  202  can be generated at the N locations on PL layer  205  and measured by static photoluminescence measurement assembly  420  and/or transient photoluminescence measurement assembly  430  without repositioning movable stage  710 . In such an embodiment, movable stage  710  is typically configured to translate substrate  203  along a single direction perpendicular to the direction of incident excitation light  201 , i.e., either vertically or horizontally. 
     In some embodiments, an OLED monitoring system includes one or more optical-fiber-based components. One such embodiment is illustrated in  FIG.  8   .  FIG.  8    is a schematic illustration of a fiber-based OLED monitoring system  800 , configured according to various embodiments of the present disclosure. OLED monitoring system  800  includes one or more optical-fiber-based components that interact with and/or direct excitation light  201 , PL emission  202 , or both excitation light  201  and PL emission  202 . 
     In some embodiments, OLED monitoring system  800  includes an array  810  that includes a plurality of probes  821 . Each of probes  821  is coupled to a fiber optic splitter  802  via a fiber bundle  822 , where each fiber bundle  822  includes at least one fiber (not shown) for directing excitation light  201  to a probe  821  and at least one fiber (not shown) for directing PL emission  202  from the probe  821 . In addition, fiber optic splitter  802  is coupled to laser  401  via an optical fiber  823  and a PL measurement assembly  830  via an optical fiber  824 . Thus, each of probes  821  is coupled to laser  401  and PL measurement assembly  830  via fiber optic splitter  802  and various optical fibers. In some embodiments, PL measurement assembly  830  includes the functionality of static PL measurement assembly  420 , transient PL measurement assembly  430 , or a combination of both. 
     Each of probes  821  is configured to direct excitation light  201  to a particular measurement location  803  on substrate  203 . In addition, each of probes  821  is configured to receive PL emission  202  and transmit PL emission  202  to PL measurement assembly  830  via fiber optic splitter  802 . 
     In the embodiment illustrated in  FIG.  8   , array  810  is a linear array that is configured to facilitate PL measurements from one edge of substrate  203  (e.g., a top edge  804 ) to an opposing edge of substrate  203  (e.g., a bottom edge  805 ) without repositioning substrate  203  via movable stage  710 . In other embodiments, array  810  extends across a portion of substrate  203  rather than from top edge  804  to bottom edge  805 . In some embodiments, array  810  is a two-dimensional array of probes  821  rather than a linear array of probes. Furthermore, in some embodiments, array  810  is disposed within a vacuum chamber of the deposition system that deposits one or more PL layers on substrate  203 . Alternatively, in some embodiments, array  810  is disposed outside of such a deposition system. In such embodiments, probes  821  are each configured to direct excitation light  201  onto substrate  203  through a respective window in the deposition system and to receive PL emission  202  through the respective window of the deposition system. 
     In some embodiments, each of probes  821  includes more than a single optical fiber for directing excitation light to substrate  203  and/or more than a single optical fiber for directing PL emission  202  from substrate  203  to PL measurement assembly  830 . One such embodiment is illustrated in  FIG.  9   .  FIG.  9    is a schematic cross-sectional view of a probe  821  of array  810 , according to various embodiments of the present disclosure. The cross-sectional view of  FIG.  9    is taken at section A-A in  FIG.  8   . In the embodiment illustrated in  FIG.  9   , probe  821  includes an emission-receiving fiber  901  and a plurality of excitation-light-transmitting fibers  902  arranged around emission-receiving fiber  901 . In alternative embodiments, probe  821  includes multiple emission-receiving fibers  901 . Probes  821  can include any other technically feasible configuration of emission-receiving fibers  901  and excitation-transmitting fibers  902  without exceeding the scope of the present disclosure. 
     According to various embodiments, an OLED monitoring system can determine one or more film characteristics of a PL material formed on a substrate within a system that has deposited the PL material.  FIG.  10    is a flow chart of process steps for determining a film characteristic of the PL material, according to various embodiments of the disclosure. 
     A method  1000  begins in step  1001 , in which light source  220  generates excitation light  201 . 
     In step  1002 , one or more components of optical assembly  240  direct excitation light  201  onto PL layer  205  formed on substrate  203 , while substrate  203  is disposed in deposition system  290 . The one or more components of optical assembly  240  may include free-space optical elements, fiber-based optical elements, or a combination of both. 
     In step  1003 , in response to excitation light  201  interacting with PL layer  205 , one or more components of optical assembly  240  direct PL emission  202  to detector  230 . The one or more components of optical assembly  240  may include free-space optical elements, fiber-based optical elements, or a combination of both. 
     In step  1004 , detector  230  receives PL emission  202 . In some embodiments, PL emission  202  includes a transient PL emission that decays over time, and in some embodiments, PL emission  202  includes a static PL emission that has a substantially a constant magnitude over time. 
     In step  1005 , detector  230  generates a signal based on PL emission  202 . In some embodiments, the signal includes static spectral intensity information, i.e., a static PL intensity for each of a plurality of wavelengths or wavelength bands. Alternatively or additionally, in some embodiments, the signal includes static spectral intensity information for a specific wavelength or wavelength band. In some embodiments, the signal includes time decay information of PL emission  202 , for example in response to an excitation pulse from light source  220 . In such embodiments, time decay information can be based on a single wavelength or wavelength band, or on a plurality of wavelengths or wavelength bands. In such embodiments, the measurement time is generally significantly longer when the time decay information is for each of a plurality of wavelengths or wavelength bands. 
     In step  1006 , computing device  250  receives the signal generated in step  1005  from the detector. 
     In step  1007 , computing device  250  determines one or more characteristics of PL layer  205  based on the signal received in step  1006 , such as a thickness of PL layer  205  and/or a dopant concentration of PL layer  205 . As set forth above, in some embodiments, computing device  250  can determine a dopant concentration of PL layer  205  based time decay information of PL emission  202  in the signal generated by detector  230 . In some embodiments, computing device  250  can determine a thickness of PL layer  205  based on a dopant concentration (determined based on time decay information of PL emission  202 ) and on static spectral intensity information of PL emission  202 . In some embodiments, computing device  250  can determine a dopant concentration of PL layer  205  based on static spectral intensity information of PL emission  202  and thickness information for PL layer  205 . For example, the thickness information may be determined via reflectometry or some other technique that can be coupled together with the embodiments for PL metrology for OLED layers. 
     In some embodiments, computing device  250  determines one or more characteristics of PL layer  205  based on a plurality of measurements of PL emission  202 , where each measurement is associated with a different measurement location. In such embodiments, computing device  250  can further determine a thickness uniformity of PL layer  205  and/or a dopant concentration uniformity of PL layer  205 . 
     Aspects of the present embodiments may be embodied as a system, method or computer program product. Accordingly, aspects of the present disclosure may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present disclosure may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
     Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.