Research on Framework Model of Real-time Scheduling System for Cluster Tool Controller Rui Lu 1, Lin-ying Li2 1 2 Police Information Department, Liaoning Police Academy, Dalian, China School of Software, Dalian University of Foreign Languages, Dalian, China ([email protected]) Abstract - On the basis of research on CTC (Cluster Tool Controller) software framework and SEMI (Semiconductor Equipment and Materials International) standard, the realtime scheduling system model of CTC, which consists of supervisory control level and module management level, is proposed. The supervisory control level is an abstract one, and is in charge of controlling schedule logic. According to principle of separating logical and function, a scheduling control logical model based on Extended Finite State Machine is proposed, as well as its control procedure under normal and exception conditions. In level of model management, task is breakdown in accordance with SEMI CTMC standard. At last, by analysis of test and verification of real-time scheduling system, the proposed model is verified using the idea of “virtual control”. Keywords - Semiconductor manufactory, cluster tool controller, real-time scheduling system, cluster tool module communications, extended finite state machine I. INTRODUCTION Cluster tools combine several single-wafer process modules and transport modules such as robots, and are widely used in semiconductor manufacturing industry and LCD production line. CTC is distributed control software system that communicates with the local module controllers (MCs), monitors events and state changes at the component modules, determines the scheduling and control commands in real time, and sends control commands to the MCs. Real-time scheduling system, which is in charge of monitoring information of equipments, as well as managing and coordinating resource of modules according to standard communication protocols, is a key part of CTC. Process Module Controller (PMC) and Transport Module Controller (TMC) are distributed entry that provides with SEMI (Semiconductor Equipment and Materials International) standard services and control wafer processing and transferring function. In other words, real-time scheduling system is a superior conductor, and be in responsible for acquiring working information of each module controller and giving proper control command. There have been some relative researches on CTC real-time scheduling system. Lee et al.  proposed a CTC scheduling system framework based on field bus technology. Unfortunately, this module does not meet SEMI standard. TrackSim is applied to Track to evaluate ____________________ Supported by Program for Liaoning Excellent Talents in University (LJQ2011132) performance of different input and dispatching rules [2, 3]. Shin et al.  presented a CTC system real-time scheduling system framework and proposal methods for the exception of equipment failure and communication delay. ClusterSim system provides with detail statistical reports. ToolSim system is ClusterSim’s upgraded version, and be applied to estimate the throughput of chemical vapor deposition (CVD) at Texas Instruments’ DMOS 5 wafer fabrication. The Single Wafer Processing (SWP), which is developed by Samsung Company, is a real-time scheduling ways to test temporary wafer processing . Huang et al.  proposed CTC control software based on Petri net for the use of remote diagnosis and advanced process control. However, they did not analyze scheduling control logic. All the researches mentioned above do not coincide with SEMI standard and its effects have great differences with actual conditions. The rest of this paper is organized as follows. In section II, the framework model of the real-time scheduling system is presented. The supervisor control layer is designed based on the extended finite state machines in section III. In section IV, it is implemented for the module management layer based on cluster tools module communications. System validation and test is addressed in section V. Lastly the conclusions are given. II. THE FRAMEWORK MODULE OF REAL-TIME SCHEDULING SYSTEM A. Cluster Tools Fig. 1 Cluster tools Cluster tools combine several single-wafer process modules and one wafer transport module. Process modules are used to process wafers, and transport modules are responsible for transferring wafers between process modules and cassette modules. Process modules can be set to difference procedure, and also be easy to change. Process flow can be changed according to different wafers. Therefore, cluster tools are referred as reconfigurable manufacturing system. Cluster tools are similar to m-stage no-wait flow shop. Same types wafers are transferred into input cassette, and then be picked by robot into process modules after being located by video system. Wafers are processed in sequence and be cooled, at last be transferred back to output cassette. Fig.1 shows cluster tools with four process modules and one dual-arm robot. B. Real-time Scheduling System Framework Fig. 2 Real-time Scheduling System Framework Model The research and development of CTC control software is a complex engineering system. It requires cross development of several subjects, such as control, mechanics, electronics and informatics. The propose is to effectively control and schedule entire system, which is the important and difficult point of system development and realization. In this paper, the function of scheduling and control in different hierarchy is separated and unified into a real-time scheduling system framework model, as showed in Fig.2. According to different hierarchy, the model has a supervisor control layer and a module management layer. The supervisor control layer determines scheduling command by means of finite state machine and then reacts to returned events. The module management layer is used to transform command of superior layer into several concrete commands that coincide with CTMS standard. The test and verification of real-time scheduling system uses various module controllers. This layer should applied simulation technique to simulate actual process of TMC, PMC and Carrier Module Controller (CMC). The PMC is responsible for controlling process device and pallet, TMC robot and slot valve, CMC control cassette and atmospheric robot. III. THE SUPERVISOR CONTROL LAYER OF REALTIME SCHEDULING SYSTEM Supervisor control layer is realized by real-time scheduling control logic of CTC control software. The scheduling control logic reads task table so as to determine control commands. Task table consists of transport task and process task which is generated by scheduling decision. According to process recipe and wafer flow model, process engineer calls algorithm exiting in scheduling algorithm library, and generates scheduling decision. The function of scheduling control is described as follows: (1) In order to make modules coordinate smoothly and to process wafer according to assigned recipe, the superior control commands are determined for every state change of system or every event aroused by module controller. (2) Determining the sequence and time of place, pick and movement. When process module or transport module finished a task, scheduling command is send to module controller through module management layer. (3) Processing the control process of events. The events include breakdown of parallel module, random auto-cleaning of PM, overrunning of hold-up time. The supervisor control layer uses finite state machine to describe scheduling control process of devices. It helps to separate logic control and function (protocol) of device, so as to made device be reconfigurable, and to satisfy the requirement of recipe and frequently change of wafer flow model. A. Extended Finite State Machine Finite state machine consists of finite states and its mutual transfer, and only be in one certain state at anytime. State machine generates an output when it received an input event, at same time, it is possible to change to other states .For a complex system, such as CTC control software, there will be thousands of states if it applies the above FSM model, and results to low effectiveness and difficulty of verification and maintenance. The hierarchical FSM is organized hierarchically by a serial of basic FSM. One or several inferior FSM corresponds to one state of superior FSM. When FSM was in a certain state, one or several inferior FSM may process in parallel or sequence way. It can realize structured and hierarchical expression of system behavior by means of using hierarchical FSM to model behavior. In order to solve the above problems and to effectively describe dynamic redundant behavior of CTC process and transport model, EFSM model , which is extended from traditional hierarchical FSM, is generated. Due to introducing inter-parameter and adding meaning of transition function, it is not difficult to avoid increasing of states caused by complex system function, so as to effectively increase the number of states and to abbreviate state explosion. B. The Scheduling Control Logical Model based on EFSM The scheduling procedure of real-time scheduling system is coordinating relationships between process model and transport model. The process task of process model depends on transport robot’s placing and picking task. Therefore, the act of transport model plays a dominant role in the modeling strategy and determines acts of process model. In order to utilize EFSM to model periodic cycle process of process model, the state number of EFSM is decreased, and complex of EFSM model is simplified; information table is also introduced to record parameters of wafer processing. When the system recipe is changed, such as adding or cutting device, introducing new recipe or control algorithm, EFSM updates processing state by means of searching new task information table, so as to avoid duplicating modeling, and then realize reconfigurable. For example, the process step 10 in Table I consists of two process model, PM1,1 and PM1,2. Its process recipe is Recipe A, while auto-cleaning recipe is Clean Recipe A. The next transport object of dual-arm robot is wafers in PM1,2 ( ○ represents the wafer in processing, while ● shows the next wafer to be transport). The process phase (consists three state: initial, steady and final) is in the state of steady. The pre-state of step 10 is in initial. TABLE I TASK TABLE Process step 10 11 12 13 …… Process model number Phase recipe PM1,1○；PM1,2 ● PM2,1○ PM3,1○ PM4,1○ …… Steady Steady Steady Steady …… A B C D …… Clean recipe #A #B #C #D …… According to schedule resources, process procedure is classified as process model, transport model and wafer. As the processing object of process and transport model, the state of wafer in different process steps is only determined by state machine of process and transport model. Therefore, wafer state machine is not considered by realtime scheduling system. Here is description of state machine of process and transport model. Fig.3 (a) shows EFSM model of process model. When EFSM model received pick completed event (Ev_Pick_Completed), model is in the state of ready (Camber Ready); when EFSM model received place completed event or swap dual-arm completed event, model is in the state of Wafer Ready. Then process model downloads recipe from CTC recipe space by recipe executor, and starts wafer process. The wafer will wait for picking by robot after being completed (Ev_Process_Completed). It can be seen from task information table that the current system is in the state of steady. The state machine of process model starts process wafer periodically. (a) EFSM model of process module (b) EFSM model of transport module Fig.3 Scheduling control logic model Fig.3 (b) shows state machine model of transport model. When system is in steady state, robot applies swap operation to complete wafer transport task. When it is in the state of initial or final, robot uses picking or placing operation to transport wafer. That is to say, transport task is completed by means of pull strategy of single-arm robot. Shin et al.  give scheduling control logic of CTC control software based on FSM. Unfortunately, only the situation of steady state, not the state of initial and final, is considered by them. There are many exceptional situations, such as malfunction of process model, auto-cleaning, temporary wafer processing. The exception handling function is embedded in the state of EFSM model. It is possible to handle accident situation according to defined procedure, and to determine whether to re-schedule system. IV. THE MODULE MANAGEMENT LAYER OF REAL-TIME SCHEDULING SYSTEM BASED ON CTMC STANDARD Scheduling control logic model produces superior control command (task) which will be separated to scheduling tasks. Module management layer is logically separated into two levels: (1) separating superior task into more detailed scheduling task; (2) communicating with inferior model control layer according to CTMC standard. A. The Procedure of Task Separating (a) Process module task Ready Cmd_Prepare PrepareStart Attribute Ev_Prepare_Completed PrepareEnd Cmd_SlotValveOpen SlotValveOpenStart Cmd_Pick Ev_SlotValveOpen_Completed Cmd_Swap SlotValveOpenEnd Function Cmd_Place PlaceStart Ev_Pick_Completed Ev_Place_Completed PlaceEnd Cmd_SlotValveClose SwapStart Ev_Swap_Completed SwapEnd Cmd_SlotValveClose TABLE II TRJOB OBJECT Name Specification ObjType Object type:TRJob TRJobID Job ID TRJobType Material type TRJobName Material identifier TRSwapObjName Identifier to be swapped TRPartner Identifier to the source TRPortID Process module indentifer TRCommand Task to be performed TRState State of transport job TRJobCreate Create TRjob object TRJobCmd Command TRJobAlert Notifcation Cmd_SlotValveClose SlotValveCloseStart Ev_SlotValveClose_Completed SlotValveCloseEnd (b) Transport module task Fig.4 The decomposition procedure of process and transport task Fig.4(a) and Fig.4(b) shows the separating procedure for process and transport task separately. Process model completes a serial of periodically operation, that is, chamber ready (chuck is moved up and be ready to accept a wafer)→wafer ready(slot valve is opened) →closed(slot valve is closed) →wafer completed(chuck is moved down and be ready to process wafer) →ready(reading process recipe) → processing(starting processing). Process model accepts control command from scheduling control logic, so as to complete every operation steps, and to send success or failure response to scheduling control logic. Transport model also completes operations: ready→wafer ready (slot valve is opened) →performing robot placing, picking operation or swapping dual-arm → closed (slot valve is closed). In Fig.4, the events or commands above symbol → consist of two or more correlative parameters, such as lot number, wafer number and process model number, et al. B. Task Operation based on CTMC SEMI E38 CTMC is communication service standard between CTC control software and model controller. It consists of material transport management (E32), object service (E30), technological process management (E40), exception management (E41), recipe management (E42) and event report (E53). CTMC defines information model, object and communication service needed by CTC monitor and coordinate module controller. In real-time scheduling system, object-oriented technique is used to design object in compliance with CTMC standard. For example, TRJob object is used to describe material transport management (shown as Table II), while PRjob object is used to describe process model management. Due to material transport management plays dominant role in the period of task execution, we only introduce TRJob object. Fig.5 The execution process of swap Fig.5 shows procedure of describing Swap command a ccording to TRJob object: TRjobCreate(“TMTRJob1”,WA FER, “002”, “001”, “PM1SwapDomain”,Swap).Req in ph ase 1 is a command sent to TMC to create transport task (“TMTRJob1”) and swap the wafer (002) being operated b y robot and the wafer (001) in process model (PM1SwapD omain); the returned event TRJobAlert（“Ready”）repres ents TMS is ready. In the same way, PMC1 in phase 2 doe s similar operation. As client, PMC1 in phase 3 start handshake confirm to server TMC1 to determine whether wafer has been placed or picked by robot. At last, TMC and PM C1 send messages (“TRJobComlete”) to CTC separately. CTMC standard is high-level application protocol. TCP/IP layer applies SECSII/HSMS protocol to complete low level data communication. The content in every TRJob member function will be coded as SECSII protocol format, and then be sent to module controller. Module controller is charge of decoding protocol data to format of CTMC object. Subsequently, module controller executes command then returns response event. V. TESTING AND VERIFICATION The meaning of virtual control is used to test and verify the real-time scheduling system. It needs to complete functions as followed: (1)developer is able to designate test scenario of operation machining cell and transport cell; (2) like actual operation of PMC and TMC controller program, simulation model management program is in compliances with CTMC standard, and construct test and verify system according to integrate communication module management program and realtime scheduling system; (3) module management has CTMC communication interface; (4) providing proper analysis tools to satisfy developer to recognition error, analyzing and statistic from different aspect. industrial computer that exists analysis program, so as to generate event log. VI. CONCLUSION On the basis of research on SEMI standard, a framework model of real-time scheduling system based on extended finite machine is presented in this paper. The proposed model consists of two layers: supervisor control layer and management layer. The character and development process of test and verification of CTC control software is described. Testing scenario, module controller and analysis tools are generated to realized the principle of “virtual control”. Thereby, verification workload is decreased in the period of development. Error detection in later period and development cycle of system are also decreased too. So the proposed system has practical significance and application value. REFERENCES Fig.6 Testing and verification system Fig .6 shows the test and verification system developed according to principle of virtual control. The details of every part are described as followed. (1) Testing scenario: users could define wafer process pattern and number of processing wafer, and simulate the scheduling process for a batch of wafers, or sent command to model controller by means of manpower. These commands consist of processing command, transport command, downloading recipe command, exception exist and stop command, et al. (2) Module management program: according to different communication objects, the finite state machine driven by network event could provide automatically connection, time-out disconnection for communication client and server; and decode SECSII format sent by CTC to message event needed by module controller, change state of module controller machine, and return message as format of SECSII. (3) PMC and TMC simulation program: simulation object create model by means of UML state diagram and sequence diagram. 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