Patent Publication Number: US-10788806-B2

Title: Initializing individual exposure field parameters of an overlay controller

Description:
BACKGROUND 
     1. Field of the Disclosure 
     Generally, the present disclosure relates to the field of semiconductor processing, and, in particular, to techniques for initializing individual exposure field parameters of an overlay controller. 
     2. Description of the Related Art 
     Integrated circuits formed on semiconductor wafers typically include a large number of circuit elements, which form an electric circuit. In addition to active devices such as, for example, field effect transistors and/or bipolar transistors, integrated circuits may include passive devices such as resistors, inductors and/or capacitors. 
     The formation of IC structures on a wafer is usually facilitated by lithographic processes used to transfer a pattern of a reticle (mask, both terms are used interchangeably herein) to a wafer. Patterns may be formed from a photoresist layer disposed on the wafer by passing light energy through a mask having an arrangement to image the desired pattern onto the photoresist layer. As a result, the pattern is transferred to the photoresist layer. In areas where the photoresist is sufficiently exposed, and after a development cycle, the photoresist material becomes soluble such that it may be removed in order to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal-containing layer, a dielectric layer, a hard mask layer, etc.). Portions of the photoresist layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer during further processing of the wafer (e.g., etching exposed portions of the underlying layer, implanting ions into the wafer, etc.). Thereafter, the remaining portions of the photoresist layer may be removed. 
     One of the key parameters in the photolithography process involves accurate overlay positioning, i.e., the process of aligning pattern features in a current layer to previously-patterned features in a previously-formed layer. Overlay is traditionally measured with relatively large test structures which are located in the scribe lines located between production die formed on a semiconductor wafer. An overlay controller provides correction parameters to the stepper to increase alignment accuracy. 
     A run-to-run process control strategy, commonly referred to as advanced process control (APC), has been employed to control the tool parameters of a stepper for overlay control. Such controllers typically employ a threaded approach, where each control thread represents a particular context defined by the particular process operation, product, stepper ID, reticle ID, etc. Each control thread represents a virtual controller that generates recipe parameters for the given context using a process model and measurement data (pre-process and post-process). From this information and the process model, the process controller determines a controller state or process state that describes the effect of the process or processes under consideration on the specific context and generates tool parameters for a current run of the stepper. 
     One difficulty arises when a new context arises (e.g., a new context for a new product or an existing context for which the previous metrology data is stale). The current state of the context may not reliably be estimated due to missing data or aged data. Such new contexts are typically initialized by running pilot substrates ahead in the process flow to obtain sufficient data to estimate the “actual” control state and to perform a control operation on the basis of data obtained by the pilot substrates. However, the processing of pilot substrates may be extremely costly and time consuming, thereby reducing throughput, tool utilization and, finally, profitability. Moreover, since the pilot substrates are processed without benefit of an APC controller, they often require rework. 
     The present invention is directed to a method and system that may solve, or at least reduce, some or all of the aforementioned problems. 
     SUMMARY 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
     Generally, the present disclosure is directed to techniques for initializing individual exposure field parameters of an overlay controller. One illustrative method includes initializing a first control thread having a first context associated with a first product type. A first layout of first exposure fields is defined for the first product type for processing in a stepper. The method further includes remapping a set of previous control state data for a set of control threads associated with other product types different than the first product type into the first layout. The other product types have layouts of second exposure fields different than the first layout. An initial set of control state data is generated for the first control thread associated with the first product type using the remapped previous control state data. The stepper is configured for processing a first substrate of the first product type using the initial set of control state data. 
     An illustrative system disclosed herein includes a stepper for exposing a substrate and an overlay controller to initialize a first control thread having a first context associated with a first product type. A first layout of first exposure fields is defined for the first product type for processing in a stepper. A set of previous control state data for a set of control threads associated with other product types different than the first product type is remapped into the first layout, wherein the other product types have layouts of second exposure fields different than the first layout. An initial set of control state data for the first control thread associated with the first product type is generated using the remapped previous control state data, and the stepper is configured for processing a first substrate of the first product type using the initial set of control state data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
         FIG. 1  is a block diagram of a system for performing exposure operations on wafers by measuring wafer distortion and correcting overlay based on the wafer distortion, in accordance with some embodiments; 
         FIG. 2  is a simplified diagram of a prior art wafer illustrating exposure fields; 
         FIGS. 3A and 3B  illustrate the remapping of exposure fields between different product types, in accordance with some embodiments; and 
         FIG. 4  is a flow diagram of one illustrative method for initializing an overlay controller, in accordance with some embodiments. 
     
    
    
     While the subject matter disclosed herein is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Various illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     The present disclosure will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present disclosure with details which are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary or customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition shall be expressively set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
       FIG. 1  is a block diagram of a system  100  for performing resist operations on substrates  105 A,  105 B (e.g., wafers), in accordance with some embodiments. The system  100  includes photoresist tools  110 A,  110 B for applying a photoresist layer to the substrates  105 A,  105 B and steppers  115 A,  115 B for patterning the photoresist layer using a reticle. For a given process operation (e.g., layer), different reticles may be used (e.g., reticles R 1 , R 2 ). The steppers  115 A,  115 B are controlled by an overlay controller  120  using an APC algorithm. Metrology tools  125 , for instance optical instruments for estimating a line width of resist features, are provided and operatively connected to the overlay controller  120  so as to provide measurement results. Thus, the overlay controller  120  and the metrology tools  125  establish a feedback control loop, in which current tool parameter settings for the steppers  115 A,  115 B are calculated on the basis of previously processed substrates. 
     Other metrology data may be provided by the stepper  115 A,  115 B itself. When a substrate  105 A,  105 B is loaded into a stepper  115 A,  115 B, alignment measurements may be taken using alignment marks present on the wafer that were formed by previous process operations. These alignment marks provide information regarding the previous layers of the substrate  105 A,  105 B. The raw alignment measurements may be laser scans of gratings of different pitches that create an interference pattern. 
     The dashed arrows in  FIG. 1  represent the different possible flow paths of the substrates  105 A,  105 B though the tools  110 A,  110 B,  115 A,  115 B, and the different reticles R 1 , R 2  that may be used. The overlay controller  120  employs control threads for each unique context, i.e., each combination of previous tool history (e.g., photoresist tool  110 A,  110 B, stepper  115 A,  115 B), product (e.g., substrate  105 A,  105 B), reticle R 1 , R 2 , etc. The overlay controller  120  calculates correction factors for the particular stepper  115 A,  115 B based on the unique context of the operation being performed and the control state associated with the context. 
     In a general process control scenario, a current control state is generated from metrology data associated with a particular one of the incoming substrates  105 A,  105 B. For example, the overlay controller  120  may compile alignment mark measurements across the entire substrate  105 A or  105 B to generate translation, rotation, and magnification correction factors. The correction factors represent the current state for the current substrate  105 A or  105 B. Techniques for generating the correction factors are known to those of ordinary skill in the art, so they are not described in greater detail herein to avoid obscuring the present subject matter. The current control state is combined with the previous control state in the overlay controller  120  to generate an updated control state. Various techniques, such as an exponentially weighted moving average Kalman filtering, maximum likelihood, etc., may be used to combine the current and previous control states. 
     However, in the case of a new thread that lacks measurement data due to missing or old measurement data, the previous control state may be missing or incomplete. Instead, measurement data from other threads may be used to populate the thread. The general equation for thread reconstruction is:
 
 AB=Y,  
 
where A represents the particular context, B represents the control state estimates, and Y represents the measurement data associated with the context. Measurement data from other contexts (e.g., same photoresist tool  110 A,  110 B and stepper  115 A,  115 B, but different substrate  105 A,  105 B) may be used to calculate the previous control state for the new thread so that it may be combined with the current control state to generate an updated control state, and the updated control state may be used to generate a control action for the associated stepper  115 A,  115 B.
 
       FIG. 2  depicts an illustrative embodiment of the two substrates  105 A,  105 B that may be subjected to exposure processes in the steppers  115 A,  115 B. A plurality of die  130 A,  130 B are formed above each wafer  105 A,  105 B. The die  130 A,  130 B define the area of the substrates  105 A,  105 B where production integrated circuit devices, e.g., microprocessors, ASICs, memory devices, will be formed. The size, shape and number of die  130 A,  130 B per substrate  105 A,  105 B depend upon the type of device under construction and the size of the substrate  105 A,  105 B. For example, the die  130 A are smaller than the die  130 B. Each substrate  105 A,  105 B may have an alignment notch  140 A,  140 B that is used to provide relatively rough alignment of the substrate  105 A,  105 B in the steppers  115 A,  115 B. 
     The exposure process performed on each substrate  105 A,  105 B is typically performed on a flash-by-flash basis as the substrate  105 A,  105 B is moved, or stepped, relative to a light source. During each step, the light source (not shown) in the stepper  115 A,  115 B projects light onto a given area of the substrate  105 A,  105 B, i.e., each flash is projected onto an exposure field  145 A,  145 B. The size of each exposure field  145 A,  145 B, as well as the number of die  130 A,  130 B within each exposure field  145 A,  145 B, may vary greatly by product. For example, the exposure field  145 A includes six die  130 A in a 2×3 pattern, while the exposure field  145 B includes four die  130 B in a 2×2 pattern. 
     In overlay control, individual correction factors may be provided for each exposure field  145 A,  145 B, referred to as correction per exposure (CPE) fields. Example CPE fields include: 
     CPE_WTRX: x translation 
     CPE_WTRY: y translation 
     CPE_FMAG: magnification (field magnification) 
     CPE_AMAG: asymmetric magnification 
     CPE_FROT: rotation 
     CPE_AROT: asymmetric rotation 
     Since the products associated with the substrates  105 A,  105 B have different numbers, sizes, and positions of the exposure fields  145 A,  145 B, the CPE state data associated with the context that processed the substrate  105 A does not directly correspond to the CPE state data associated with the context that processed the substrate  105 B. 
     To allow the use of CPE state data from other threads, the CPE data from other product types is remapped to the exposure field space of the current product type associated with the new thread.  FIGS. 3A, 3B  illustrate the correspondence between the exposure fields  145 A of Product A associated with the substrate  105 A and the exposure fields  145 B of Product B associated with the substrate  105 B. The value for the particular CPE parameter represented by data point  300  for an exposure field  145 A may be estimated using the values of the CPE parameter for the nearby exposure fields  145 B. A mapping procedure is performed to map the data values for the CPE parameters from the Product B exposure field space to the Product A exposure field space. 
     In one embodiment shown in  FIG. 3A , the data point  300  for the CPE parameter of a Product A exposure field  145 A may be determined using an average of the CPE parameter for the k nearest neighbors (shown in bold). 
     
       
         
           
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     In another embodiment shown in  FIG. 3B , the data point  300  for the CPE parameter may be determined using a distance weighted average based on all other fields or all fields within a given radius. The length of an arrow  310  is the distance d. The weight of the measured value of an exposure field  145 B decreases with distance. For example, an exponential discounting factor, w, may be used:
 
 w=e   −md     2   ,
 
where m is a tuning parameter that governs how quickly the data is discounted. This function is known as a Gaussian radial basis function (RBF). If a particular radius is employed, data points with distances greater than a predetermined threshold may be ignored.
 
     In yet another embodiment, the CPE data for the exposure fields  145 B may be fit to a model, such as a polynomial model. The model may be employed to generate an estimated value for the exposure field  145 A based on the location of the data point  300 . 
     In some embodiments, the scan direction may also be used to filter the previous CPE state data. It has been noted that the y-translation factor (CPE_WTRY) correction factor is affected by scan direction (i.e., up to down or down to up). Thus, only CPE_WTRY data associated with the same scan direction as the exposure field  145 A is used to generate the data point  300 . 
     By incorporating the CPE state data from other threads and remapping the exposure fields, the previous control state for the current context may be estimated. The state estimate contributions come from the thread component matrix A:
 
 {circumflex over (B)}=A   −   Y,  
 
where each row of Y corresponds to an existing thread and A +  represents an inversion or pseudo-inversion of the A matrix.
 
     The state estimate may be represented as:
 
 Ŷ=A   new   {circumflex over (B)} 
 
 Ŷ =( A   new   A   + ) Y  
 
 Ŷ=ωY  
 
     Hence, the state estimates from the remapped CPE data are weighted by the sum of the observed states (ω). The influence that each measurement has on the predicted value is called the influence vector (or projection vector) ω. If a measurement has a large influence on the predicted state, then it has a large magnitude. In contrast, measurements with a small influence on the predicted state have a small magnitude. 
     The state estimate, Ŷ, generated using CPE data from other threads may be used as the basis for generating a control action for the current substrate  105 A,  105 B. The current state generated using the metrology data from the current substrate  105 A may be combined with the state estimate, Ŷ, to update the control state and generate the control action. 
       FIG. 4  is a flow diagram of one illustrative method  400  for initializing an overlay controller  120 , in accordance with some embodiments. The elements of the method  400  may be performed by the overlay controller  120 . 
     In method block  405 , a first control thread is initialized. The first control thread may have no previous control state data or incomplete control state data. The first control thread has a first context associated with a first product type. A first layout of first exposure fields  145 A is defined for the first product type. 
     In method block  410 , control state data for a set of control threads associated with product types other than the first product type that have layouts of second exposure fields different than the first layout are remapped. 
     In method block  415 , an initial set of control state data is generated for the first control thread using the remapped control state data. Each of the first exposure fields is associated with a subset of the initial set of control state data. For example, the subset may include the correction factors associated with the exposure field. 
     In method block  420 , the alignment of a first substrate is measured in the stepper  115 A,  115 B. 
     In method block  425 , a current control state for the first substrate is generated based on the alignment data. For example, the current control state may include the correction parameters for each exposure field calculated based on the measured alignment. 
     In method block  430 , the current control state may be combined with the initial control state to generate an updated control state. Various techniques, such as an exponentially weighted moving average, a Kalman filter, a maximum likelihood approach, etc., may be used to combine the current and previous control states. 
     In method block  435 , the stepper  110 A,  110 B is configured using the updated control state. 
     In method block  440 , the first substrate is exposed in the stepper  110 A,  110 B. 
     The remapping of the CPE data from other products described herein allows new control threads to be initialized, even when the layout of exposure fields is different. The initialization allows the overlay controller  120  to generate a control action for the first substrate in the new control thread without requiring the use of pilot wafers. This approach increases throughput and quality, while also reducing rework. 
     In some embodiments, certain aspects of the techniques described above may be implemented by one or more processors of a processing system executing software. The method  400  described herein may be implemented by executing software on a computing device, such as the overlay controller  120  of  FIG. 1 , however, such methods are not abstract in that they improve the operation of the stepper  110 A,  110 B and the quality of the fabricated substrates  105 A,  105 B. Prior to execution, the software instructions may be transferred from a non-transitory computer readable storage medium to a memory. 
     The software may include one or more sets of executable instructions stored or otherwise tangibly embodied on a non-transitory computer readable storage medium. The software can include the instructions and certain data that, when executed by one or more processors, manipulate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer readable storage medium can include, for example, a magnetic or optical disk storage device, solid state storage devices such as Flash memory, a cache, random access memory (RAM) or other non-volatile memory devices, and the like. The executable instructions stored on the non-transitory computer readable storage medium may be in source code, assembly language code, object code, or other instruction format that is interpreted or otherwise executable by one or more processors. 
     A computer readable storage medium may include any storage medium, or combination of storage media, accessible by a computer system during use to provide instructions and/or data to the computer system. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-Ray disc), magnetic media (e.g., floppy disc, magnetic tape or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer readable storage medium may be embedded in the computing system (e.g., system RAM or ROM), fixedly attached to the computing system (e.g., a magnetic hard drive), removably attached to the computing system (e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the computer system via a wired or wireless network (e.g., network accessible storage (NAS)). 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Note that the use of terms, such as “first,” “second,” “third” or “fourth” to describe various processes or structures in this specification and in the attached claims is only used as a shorthand reference to such steps/structures and does not necessarily imply that such steps/structures are performed/formed in that ordered sequence. Of course, depending upon the exact claim language, an ordered sequence of such processes may or may not be required. Accordingly, the protection sought herein is as set forth in the claims below.