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
(luruilly@sina.com)
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. [1] 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. [4] 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 [5].
Huang et al. [6] 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 [7][8].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 [9][10], 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. [4] 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. When it received control command
from CTC, or internal events of processing complete/
alarm, simulation object changes its state according to
protocol standard. PMC and TMC simulation objects have
same parent class. The only difference is number function
and internal logic after extended from parent class.
(4) Analysis tools: PMC and TMC simulation
program trace event message driven by a certain of
scenario. Messages are stored in relevant tables in
Microsoft SQL, and be sent by Socket interface to
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