Patent Publication Number: US-2020279783-A1

Title: Process control of semiconductor fabrication based on linkage between different fabrication steps

Description:
BACKGROUND 
     Metrology is typically employed during fabrication of structures on semiconductor wafers in order to monitor and control the fabrication process. Measurements of structural characteristics, such as critical dimension (CD), sidewall angle (SWA), height, and trench depth taken at various processing steps provide information such as whether or not a processing step produces an acceptable result, as well as metrics such as etch rate and deposition rate. Such measurements that are taken at a later processing step when manufacturing a given wafer often indicate a problem that could have been corrected at an earlier processing step. While this information may be used to adjust the earlier processing step for subsequently fabricated wafers, this is too late for the given wafer if the problem results in a defect in the given wafer that cannot subsequently be corrected. 
     SUMMARY 
     In one aspect of the invention a computer-implemented method is provided for use in process control during manufacture of semiconductor devices on semiconductor wafers, the method including collecting scatterometric spectra of a reference fin structure of a FinFET on a reference semiconductor wafer at a first checkpoint proximate to a first processing step during fabrication of the reference semiconductor wafer, collecting reference measurements of the reference fin structure at a second checkpoint proximate to a second processing step during the fabrication of the reference semiconductor wafer, where the second checkpoint is subsequent to the first checkpoint, and performing machine learning to identify correspondence between the scatterometric spectra and values based on the reference measurements, thereby training a prediction model for producing a prediction value associated with a production fin structure of the FinFET on a production semiconductor wafer based on scatterometric spectra of the production fin structure collected at a first checkpoint during fabrication of the production semiconductor wafer, where the production fin structure corresponds to the reference fin structure, and where the first checkpoint during the fabrication of the production semiconductor wafer corresponds to the first checkpoint during the fabrication of the reference semiconductor wafer. 
     In another aspect of the invention the method further includes collecting the scatterometric spectra of the production fin structure at the first checkpoint during the fabrication of the production semiconductor wafer, and producing, using the prediction model, the prediction value associated with the production fin structure based on scatterometric spectra of the production fin structure. 
     In another aspect of the invention the method further includes producing the prediction value at the first checkpoint during the fabrication of the production semiconductor wafer. 
     In another aspect of the invention the prediction value is predictive of an expected measurement of the production fin structure at a second checkpoint during the fabrication of the production semiconductor wafer corresponding to the second checkpoint during the fabrication of the reference semiconductor wafer. 
     In another aspect of the invention the method further includes comparing the expected measurement with a predefined target measurement planned for the production fin structure at the second checkpoint during the fabrication of the production semiconductor wafer, and adjusting a process control parameter of a processing step subsequent to the first checkpoint during the fabrication of the production semiconductor wafer and prior to the second checkpoint during the fabrication of the production semiconductor wafer, to reduce a difference between the expected measurement and the predefined target measurement. 
     In another aspect of the invention the comparing includes comparing at the first checkpoint during the fabrication of the production semiconductor wafer. 
     In another aspect of the invention the adjusting includes providing input to a semiconductor manufacturing tool for controlling operation of the semiconductor manufacturing tool during the fabrication of the production semiconductor wafer. 
     In another aspect of the invention the performing machine learning includes identifying the correspondence between the scatterometric spectra and the values based on the reference measurements where predefined statistical criteria are met indicating that any of the scatterometric spectra of the reference fin structure at the first checkpoint are statistically linked to any of the values based on the reference measurements of the reference fin structure at the second checkpoint. 
     In another aspect of the invention the fabrication of the production semiconductor wafer and the production fin structure uses a process identical to a process used to fabricate the reference semiconductor wafer and the reference fin structure, where the first and second checkpoints during the fabrication of the production semiconductor wafer correspond, respectively, to the first and second checkpoints during the fabrication of the reference semiconductor wafer. 
     In another aspect of the invention the method further includes determining, using the scatterometric spectra of the production fin structure, a height difference between a top of the production fin structure and a top of a silicon oxide layer above a trench adjacent to the production fin structure, calculating a total etch amount by adding the expected measurement to the height difference, converting the total etch amount to an etch time, and controlling one or more processing steps after the first checkpoint during the fabrication of the production semiconductor wafer to implement the etch time in order to achieve a predefined target measurement planned for the production fin structure at the second checkpoint during the fabrication of the production semiconductor wafer, where the expected measurement and the predefined target measurement are of height of the production fin structure. 
     In another aspect of the invention the method further includes comparing the expected measurement with the predefined target measurement, and adjusting the etch time to reduce a difference between the expected measurement and the predefined target measurement. 
     In another aspect of the invention the method further includes determining, using the scatterometric spectra of the reference fin structure, a height difference between a top of the reference fin structure and a top of a silicon oxide layer above a trench adjacent to the reference fin structure, where the reference measurement is of height of the reference fin structure, calculating a total etch amount by adding the reference measurement to the height difference, where the total etch amount is used as one of the values based on the reference measurements used to train the prediction model, collecting the scatterometric spectra of the production fin structure at the first checkpoint during the fabrication of the production semiconductor wafer, producing, using the prediction model, the prediction value representing a total etch amount associated with the production fin structure based on scatterometric spectra of the production fin structure, converting the total etch amount to an etch time, and controlling one or more processing steps after the first checkpoint during the fabrication of the production semiconductor wafer to implement the etch time in order to achieve a predefined target measurement at the second checkpoint during the fabrication of the reference semiconductor wafer, where the predefined target measurement is of height of the production fin structure. 
     In another aspect of the invention the method further includes determining an expected fin height at the second checkpoint from the etch time, comparing the expected fin height with the predefined target measurement, and adjusting the etch time to reduce a difference between the expected fin height and the predefined target measurement. 
     In another aspect of the invention a system is provided for use in process control during manufacture of semiconductor devices on semiconductor wafers, the system including a spectrum acquisition tool configured to collect scatterometric spectra of a reference fin structure of a FinFET on a reference semiconductor wafer at a first checkpoint proximate to a first processing step during fabrication of the reference semiconductor wafer, a reference tool configured to collect reference measurements of the reference fin structure at a second checkpoint proximate to a second processing step during the fabrication of the reference semiconductor wafer, where the second checkpoint is subsequent to the first checkpoint, and a training unit configured to perform machine learning to identify correspondence between the scatterometric spectra and values based on the reference measurements, thereby training a prediction model for producing a prediction value associated with a production fin structure of the FinFET on a production semiconductor wafer based on scatterometric spectra of the production fin structure collected at a first checkpoint during fabrication of the production semiconductor wafer, where the production fin structure corresponds to the reference fin structure, and where the first checkpoint during the fabrication of the production semiconductor wafer corresponds to the first checkpoint during the fabrication of the reference semiconductor wafer. 
     In another aspect of the invention the spectrum acquisition tool is configured to collect the scatterometric spectra of the production fin structure at the first checkpoint during the fabrication of the production semiconductor wafer, and further includes a prediction unit configured to produce, using the prediction model, the prediction value associated with the production fin structure based on scatterometric spectra of the production fin structure. 
     In another aspect of the invention the prediction unit is configured to produce the prediction value at the first checkpoint during the fabrication of the production semiconductor wafer. 
     In another aspect of the invention the prediction value is predictive of an expected measurement of the production fin structure at a second checkpoint during the fabrication of the production semiconductor wafer corresponding to the second checkpoint during the fabrication of the reference semiconductor wafer. 
     In another aspect of the invention the system further includes a process control unit configured to compare the expected measurement with a predefined target measurement planned for the production fin structure at the second checkpoint during the fabrication of the production semiconductor wafer, and adjust a process control parameter of a processing step subsequent to the first checkpoint during the fabrication of the production semiconductor wafer and prior to the second checkpoint during the fabrication of the production semiconductor wafer, to reduce a difference between the expected measurement and the predefined target measurement. 
     In another aspect of the invention the training unit is configured to perform the machine learning to identify the correspondence between the scatterometric spectra and the values based on the reference measurements where predefined statistical criteria are met indicating that any of the scatterometric spectra of the reference fin structure at the first checkpoint are statistically linked to any of the values based on the reference measurements of the reference fin structure at the second checkpoint. 
     In another aspect of the invention the spectrum acquisition tool is configured to determine, using the scatterometric spectra of the production fin structure, a height difference between a top of the production fin structure and a top of a silicon oxide layer above a trench adjacent to the production fin structure, and the process control unit is configured to calculate a total etch amount by adding the expected measurement to the height difference, convert the total etch amount to an etch time, and control one or more processing steps after the first checkpoint during the fabrication of the production semiconductor wafer to implement the etch time in order to achieve a predefined target measurement planned for the production fin structure at the second checkpoint during the fabrication of the production semiconductor wafer, where the expected measurement and the predefined target measurement are of height of the production fin structure. 
     In another aspect of the invention the spectrum acquisition tool is configured to determine, using the scatterometric spectra of the reference fin structure, a height difference between a top of the reference fin structure and a top of a silicon oxide layer above a trench adjacent to the reference fin structure, where the reference measurement is of height of the reference fin structure, the training unit is configured to use a total etch amount as one of the values based on the reference measurements used to train the prediction model, where the total etch amount is calculated by adding the reference measurement to the height difference, the spectrum acquisition tool is configured to collect the scatterometric spectra of the production fin structure at the first checkpoint during the fabrication of the production semiconductor wafer, the prediction unit is configured to produce, using the prediction model, the prediction value representing a total etch amount associated with the production fin structure based on scatterometric spectra of the production fin structure, and the process control unit is configured to convert the total etch amount to an etch time, and control one or more processing steps after the first checkpoint during the fabrication of the production semiconductor wafer to implement the etch time in order to achieve a predefined target measurement at the second checkpoint during the fabrication of the reference semiconductor wafer, where the predefined target measurement is of height of the production fin structure. 
     In another aspect of the invention the process control unit is configured to determine an expected fin height at the second checkpoint from the etch time, compare the expected fin height with the predefined target measurement, and adjust the etch time to reduce a difference between the expected fin height and the predefined target measurement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which: 
         FIGS. 1A and 1B , taken together, is a simplified conceptual illustration of a system for predictive process control of semiconductor fabrication, constructed and operative in accordance with an embodiment of the invention; 
         FIGS. 2A and 2B  are simplified conceptual illustrations of a FinFET fin structure at different processing steps during FinFET fabrication, useful in understanding an embodiment of the invention; 
         FIGS. 3A and 3B  are simplified conceptual illustrations of a FinFET fin structure at different processing steps during FinFET fabrication, useful in understanding additional embodiments of the invention; and 
         FIGS. 4-6  are simplified flowchart illustrations of exemplary methods of operation of the system of  FIGS. 1A and 1B , operative in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made to  FIGS. 1A and 1B , which, taken together, is a simplified conceptual illustration of a system for predictive process control of semiconductor fabrication, constructed and operative in accordance with an embodiment of the invention. In  FIG. 1A , a spectrum acquisition tool  100 , such as a Spectral Ellipsometer (SE), a Spectral Reflectometer (SR), or a Polarized Spectral Reflectometer, is employed to collect, in accordance with conventional techniques, scatterometric spectra of a reference structure, such as fin structure  102  of a Fin Field-effect transistor (FinFET), on a reference semiconductor wafer  104 , such as by performing spectrum photometry on reference fin structure  102 . Spectrum acquisition tool  100  collects the scatterometric spectra of reference fin structure  102  at a first checkpoint proximate to a selected processing step during fabrication of reference semiconductor wafer  104 , such as just after completion of a given etch step. Any processing step may be selected for the first checkpoint provided that it is followed by one or more later processing steps. 
     A reference tool  106 , such as a Critical Dimension Scanning Electron Microscope (CD-SEM), an Atomic Force Microscope (AFM), or a Critical Dimension Atomic Force Microscope (CD-AFM), is employed to collect, in accordance with conventional techniques, reference measurements of reference fin structure  102  on reference semiconductor wafer  104  at a second checkpoint proximate to a different processing step during fabrication of reference semiconductor wafer  104 , such as just after completion of a different etch step, where the second checkpoint is subsequent to the first checkpoint during the fabrication process. The reference measurements may be any type of measurements of, or relative to, reference fin structure  102 , such as critical dimension (CD), sidewall angle (SWA), height, and trench depth. The processing step to which the second checkpoint is proximate and the processing step to which the first checkpoint is proximate may be separated by zero or more intermediate processing steps. 
     Spectrum acquisition tool  100  and reference tool  106  preferably obtain multiple scatterometric spectra and reference measurements for multiple reference fin structures  102  on one or more reference semiconductor wafers  104 . 
     A training unit  108  is configured to train a prediction model  110  by performing machine learning (ML) to identify correspondence between the scatterometric spectra and values based on the reference measurements, such as the reference measurements themselves or values derived therefrom. Training unit  108  may employ any known ML technique suitable for identifying such correspondence between the scatterometric spectra and the values based on the reference measurements where predefined statistical criteria are met indicating that particular scatterometric spectra of a given reference fin structure  102  at the first checkpoint are statistically linked to particular value based on the reference measurements of the given reference fin structure  102  at the second checkpoint. Prediction model  110  is provided for use with process control apparatus configured to control manufacture of semiconductor devices on semiconductor wafers, as is now described with reference to  FIG. 1B . 
     In  FIG. 1B , a spectrum acquisition tool  100 ′, which may be spectrum acquisition tool  100  or another similar or identical spectrum acquisition tool, is employed during a production process, such as during a high-volume manufacturing (HVM) process of fabricating semiconductor devices on semiconductor wafers, to collect scatterometric spectra of a production fin structure  102 ′ of a FinFET on a production semiconductor wafer  104 ′, where production fin structure  102 ′ corresponds to reference fin structure  102 , and where the spectra are obtained in the manner described above with reference to spectrum acquisition tool  100  in  FIG. 1A . The process used to fabricate production semiconductor wafer  104 ′ and production fin structure  102 ′ is preferably identical to the process described in  FIG. 1A  that is used to fabricate reference semiconductor wafer  104  and reference fin structure  102 , such that the first and second checkpoints referred to in the description of  FIG. 1A  correspond, respectively, to the same points during the process used to fabricate production semiconductor wafer  104 ′ and production fin structure  102 ′. 
     In  FIG. 1B  spectrum acquisition tool  100 ′ collects the scatterometric spectra of production fin structure  102 ′ at the first checkpoint during fabrication of production semiconductor wafer  104 ′. A prediction unit  112  employs prediction model  110  to produce, preferably also at the first checkpoint, a prediction value associated with production fin structure  102 ′ at the second checkpoint during fabrication of production semiconductor wafer  104 ′. Prediction unit  112  produces the prediction value by identifying a value based on a reference measurement in prediction model  110  that corresponds to the scatterometric spectra of production fin structure  102 ′ collected at the first checkpoint, and then using the identified value as the prediction value. The prediction value is predictive of an expected measurement of production fin structure  102 ′ at the second checkpoint, but that is produced in advance of the second checkpoint, such as at the first checkpoint, during fabrication of production semiconductor wafer  104 ′. 
     A process control unit  114 , which may be any known process control hardware and/or software for controlling the process of fabricating semiconductor devices on semiconductor wafers, is configured to compare, preferably at the first checkpoint during fabrication of the production semiconductor wafer  104 ′, the expected measurement of production fin structure  102 ′ with a predefined target measurement  116  planned for production fin structure  102 ′ at the second checkpoint during fabrication of production semiconductor wafer  104 ′. Process control unit  114  is also configured to adjust, in accordance with predefined adjustment protocols, one or more process control parameters, such as etch time or deposition rate, of one or more processing steps subsequent to the first checkpoint during fabrication of production semiconductor wafer  104 ′ and prior to the second checkpoint during fabrication of production semiconductor wafer  104 ′, to reduce a difference between the expected measurement and the predefined target measurement, if such a difference is found. Process control unit  114  preferably effects such adjustments by providing, in accordance with conventional techniques, input to any known semiconductor manufacturing tool  118  (e.g., lithography tool, etch tool, deposition tool, etc.) for controlling operation of the tool during the fabrication of production semiconductor wafer  104 ′. 
     Operation of the systems of  FIGS. 1A and 1B  may be illustrated by way of example with reference to  FIG. 2A , which shows silicon fin structures  200 A- 200 B of a FinFET at a first checkpoint during their fabrication, and  FIG. 2B , which shows fin structures  200 A- 200 B at a second checkpoint subsequent to the first checkpoint during the fabrication process. In  FIG. 2A  the sidewalls of fin structures  200 A- 200 B are shown covered in a layer  202  of silicon oxide. The top of fin structures  200 A- 200 B is also capped with a layer  204  of silicon oxide, atop which sits a layer of silicon nitride  206  below yet another layer  208  of silicon oxide. Fin structure  200 A is separated from fin structure  200 B by an air-filled trench  210 , such as may be formed in accordance with conventional shallow trench isolation (STI) techniques. In  FIG. 2B , fin structures  200 A- 200 B is shown after an etch processing step has removed layers  204 ,  206 , and  208 . 
     The system of  FIG. 1A  collects, on one or more reference semiconductor wafers, scatterometric spectra of fin structures  200 A- 200 B at the first checkpoint shown in  FIG. 2A , and thereafter collects reference measurements of the critical dimension (CD) of fin structures  200 A- 200 B at the second checkpoint shown in  FIG. 2B . Using the scatterometric spectra and reference measurements, the system of  FIG. 1A  trains prediction model  110  to identify various scatterometric spectra of fin structures  200 A- 200 B at the first checkpoint that are statistically linked to various reference measurements of fin structures  200 A- 200 B at the second checkpoint. 
     At the first checkpoint during fabrication of fin structures  200 A- 200 B on a production semiconductor wafer, the system of  FIG. 1B  collects scatterometric spectra of fin structures  200 A- 200 B as shown in  FIG. 2A  and uses prediction model  110  to produce a prediction value representing an expected CD measurement of fin structures  200 A- 200 B at the second checkpoint using the production scatterometric spectra just collected at the first checkpoint. Prediction model  110  is used to produce the expected CD measurement by identifying a prediction value in prediction model  110  that corresponds to the scatterometric spectra of production fin structures  200 A- 200 B collected at the first checkpoint, and then using the identified prediction value as the expected CD measurement. The system of  FIG. 1B  then compares, at the first checkpoint, the expected CD measurement with a predefined target CD measurement planned for fin structures  200 A- 200 B at the second checkpoint. If no difference is found between the expected CD measurement and the predefined target CD measurement, the fabrication process continues to the second checkpoint without adjustment. If a difference is found, the system of  FIG. 1B  adjusts, in accordance with predefined adjustment protocols, one or more process control parameters of one or more processing steps of the production fabrication process between the first and second checkpoints, in order to reduce the difference and thereby achieve the predefined target CD measurement at the second checkpoint. 
     Operation of the systems of  FIGS. 1A and 1B  may be illustrated by way of another example with reference to  FIGS. 3A and 3B , which show silicon fin structures  300 A- 300 B of a FinFET at first and second checkpoints, respectively, during their fabrication. In  FIG. 3A  fin structures  300 A- 300 B are shown at the first checkpoint completely covered in a layer  302  of silicon oxide, where the height difference between the top of the silicon oxide layer above trench  304  and the top of fin structures  300 A- 300 B, indicated by reference numeral  306 , is referred to herein as the STI step height.  FIG. 3B  shows fin structures  300 A- 300 B at the second checkpoint after a portion of the silicon oxide has been etched away to reveal an upper portion of fin structures  300 A- 300 B, where the height difference between the top of fin structures  300 A- 300 B and the top of the post-etch silicon oxide layer in trench  304  is referred to herein as the fin height, indicated by reference numeral  306 . 
     In one embodiment, the system of  FIG. 1A  collects, on one or more reference semiconductor wafers, scatterometric spectra of fin structures  300 A- 300 B at the first checkpoint shown in  FIG. 3A , and thereafter collects reference measurements of the fin height of fin structures  300 A- 300 B at the second checkpoint shown in  FIG. 3B . The system of  FIG. 1A  then uses the scatterometric spectra and reference measurements to train prediction model  110  as described hereinabove. 
     At the first checkpoint during fabrication of fin structures  300 A- 300 B on a production semiconductor wafer, the system of  FIG. 1B  collects scatterometric spectra of fin structures  300 A- 300 B as shown in  FIG. 3A  and uses prediction model  110  in the manner described above to produce a prediction value representing an expected fin height measurement of fin structures  300 A- 300 B at the second checkpoint using the production scatterometric spectra just collected at the first checkpoint. The system of  FIG. 1B  also uses the production scatterometric spectra from the first checkpoint to determine the STI step height of fin structures  300 A- 300 B in accordance with conventional techniques, such as by spectrum acquisition tool  100 ′ employing model-based Optical Critical Dimension (OCD) scatterometry. Process control unit  114  then compares, at the first checkpoint, the expected fin height measurement with a predefined target fin height measurement planned for fin structures  300 A- 300 B at the second checkpoint. Process control unit  114  then calculates a total etch amount by adding the expected fin height measurement to the STI step height. Process control unit  114  then converts the total etch amount, in accordance with conventional techniques, to an etch time that process control unit  114  then applies to the production semiconductor wafer by controlling one or more processing steps after the first checkpoint to implement the etch time in order to achieve the predefined target fin height measurement. If no difference is found between the expected fin height measurement and the predefined target fin height measurement, the fabrication process continues to the second checkpoint without adjustment to the etch time. If a difference is found, process control unit  114  adjusts, in accordance with predefined adjustment protocols, the etch time (either directly or by first adjusting the total etch amount), in order to reduce the difference and thereby achieve the predefined target fin height measurement at the second checkpoint. 
     In an alternative embodiment, in addition to collecting, on one or more reference semiconductor wafers, scatterometric spectra at the first checkpoint shown in  FIG. 3A  and reference measurements of fin height at the second checkpoint shown in  FIG. 3B , spectrum acquisition tool  100  also uses the scatterometric spectra from the first checkpoint to determine the STI step height of fin structures  300 A- 300 B in accordance with conventional techniques, such as by employing model-based Optical Critical Dimension (OCD) scatterometry. The system of  FIG. 1A  then calculates a total etch amount by adding the reference fin height measurement to the STI step height, and uses various collected scatterometric spectra and calculated total etch amounts to train prediction model  110  to identify scatterometric spectra of fin structures  300 A- 300 B at the first checkpoint that are statistically linked to total etch amounts calculated based on reference measurements of fin structures  300 A- 300 B at the second checkpoint. 
     At the first checkpoint during fabrication of fin structures  300 A- 300 B on a production semiconductor wafer, spectrum acquisition tool  100 ′ collects scatterometric spectra of fin structures  300 A- 300 B as shown in  FIG. 3A , and prediction unit  112  uses prediction model  110  to produce a prediction value representing a total etch amount by identifying a total etch amount in prediction model  110  that corresponds to the production scatterometric spectra of production fin structures  300 A- 300 B collected at the first checkpoint. Process control unit  114  the converts the total etch amount to an etch time, from which an expected fin height at the second checkpoint is also determined, and which process control unit  114  applies to the production semiconductor wafer by controlling one or more processing steps after the first checkpoint to implement the etch time. Process control unit  114  compares, at the first checkpoint, the expected fin height measurement with a predefined target fin height measurement planned for fin structures  300 A- 300 B at the second checkpoint. If no difference is found between the expected fin height measurement and the predefined target fin height measurement, the fabrication process continues to the second checkpoint without adjustment to the etch time. If a difference is found, process control unit  114  adjusts, in accordance with predefined adjustment protocols, the etch time (either directly or by first adjusting the total etch amount), in order to reduce the difference and thereby achieve the predefined target fin height measurement at the second checkpoint. 
     Reference is now made to  FIG. 4  which is a simplified flowchart illustration of an exemplary method of operation of the system of  FIGS. 1A and 1B , operative in accordance with an embodiment of the invention. In the method of  FIG. 4 , scatterometric spectra of a reference fin structure of a Fin Field-effect transistor (FinFET) on reference semiconductor wafers are collected at a first checkpoint proximate to a selected processing step during fabrication of the reference semiconductor wafers (step  400 ). Reference measurements of the reference fin structure are collected at a second checkpoint proximate to a different processing step subsequent to the first checkpoint (step  402 ). A prediction model is trained by performing machine learning (ML) to identify correspondence between the scatterometric spectra and values based on the reference measurements (step  404 ). Scatterometric spectra of a production fin structure, corresponding to the reference fin structure, on a production semiconductor wafer are collected at the first checkpoint during fabrication of the production semiconductor wafer (step  406 ). A prediction value predictive of an expected measurement of the production fin structure at the second checkpoint is produced, preferably at the first checkpoint, using the prediction model based on the scatterometric spectra of the production fin structure collected at the first checkpoint (step  408 ). The expected measurement of the production fin structure is compared, preferably at the first checkpoint, with a predefined target measurement planned for the production fin structure at the second checkpoint (step  410 ). One or more process control parameters of any processing steps of the production fabrication process between the first and second checkpoints are adjusted to reduce any difference found between the expected measurement and the predefined target measurement (step  412 ). 
     Reference is now made to  FIG. 5  which is a simplified flowchart illustration of an exemplary method of operation of the system of  FIGS. 1A and 1B , operative in accordance with an alternative embodiment of the invention. In the method of  FIG. 5 , steps  400 - 408  of the method of  FIG. 4  are performed, where the prediction value produced in step  408  is predictive of an expected fin height measurement of the production fin structure at the second checkpoint (step  500 ). The production scatterometric spectra are used to determine, preferably at the first checkpoint, the STI step height of the production fin structure (step  502 ). The expected measurement of the production fin structure is compared, preferably at the first checkpoint, with a predefined target fin height measurement planned for the production fin structure at the second checkpoint (step  504 ). A total etch amount is calculated by adding the expected fin height measurement to the STI step height (step  506 ). The total etch amount is converted to an etch time (step  508 ). If a difference is found between the expected fin height measurement and the predefined target fin height measurement, the etch time is adjusted in order to reduce the difference and thereby achieve the predefined target fin height measurement at the second checkpoint (step  510 ). One or more processing steps after the first checkpoint are controlled to implement the etch time in order to achieve the predefined target fin height measurement at the second checkpoint (step  512 ). 
     Reference is now made to  FIG. 6  which is a simplified flowchart illustration of an exemplary method of operation of the system of  FIGS. 1A and 1B , operative in accordance with an alternative embodiment of the invention. In the method of  FIG. 6 , steps  400 - 402  of the method of  FIG. 4  are performed (step  600 ). The reference scatterometric spectra are used to determine, preferably at the first checkpoint, the STI step height of the reference fin structure (step  602 ). A total etch amount is calculated by adding the reference fin height measurement to the STI step height (step  604 ). A prediction model is trained by performing machine learning (ML) to identify correspondence between various scatterometric spectra and total etch amounts (step  606 ). Steps  406 - 408  of the method of  FIG. 4  are performed, where the prediction value produced in step  408  is predictive of a total etch amount (step  608 ). The total etch amount is converted to an etch time and an expected fin height at the second checkpoint (step  610 ). The expected measurement of the production fin structure is compared, preferably at the first checkpoint, with a predefined target fin height measurement planned for the production fin structure at the second checkpoint (step  612 ). If a difference is found between the expected fin height measurement and the predefined target fin height measurement, the etch time is adjusted in order to reduce the difference and thereby achieve the predefined target fin height measurement at the second checkpoint (step  614 ). One or more processing steps after the first checkpoint are controlled to implement the etch time in order to achieve the predefined target fin height measurement at the second checkpoint (step  616 ). 
     The flowchart illustrations and block diagrams in the drawing figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the invention. In this regard, each block in the flowchart illustrations or block diagrams may represent a module, segment, or portion of computer instructions, which comprises one or more executable computer instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in a block may occur out of the order noted in the drawing figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the flowchart illustrations and block diagrams, and combinations of such blocks, can be implemented by special-purpose hardware-based and/or software-based systems that perform the specified functions or acts. 
     The descriptions of the various embodiments of the invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. For example, the systems and methods described herein are applicable to any type of structure on semiconductor wafers. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments.