Throughput simulation Technology > Throughput simulation
In semiconductor and display device, throughput means the amount of wafer or board processing during unit time in unit process. As its unit, Wafer/Hour (WPH) or Glass/Hour (GPH) is frequently used. It is possible to predict mechanical throughput through process time of PM, speed of vacuum robot, air absorption and discharge speed of load lock, cooling/heating of glass or wafer, speed data of ATM robot, etc.
For throughput of PM, amount of PM chamber and average process time of each PM are used when up/down of lift pin used for PM is included in PM time. Its calculation formula is as follow.
Mechanical T.P of PM= No. of PM X [3600 / average process time]
Glass transfer order of vacuum robot is swap in load lock, swap in PM and rotation in the next station. In throughput simulation, average value is used for rotation. Its calculation formula is as follow.
T.P of VR = 3600 / (Swap time at LL + Swap time at PM + 2 X Rotate average)
Glass transfer order of load lock(input) is ATM robot placement in LL, ATM door close, pumping in the condition that glass is placed in LL, slot valve open, vacuum robot pick, slot valve close, vent in the condition that glass exists and ATM door valve open. Its throughput calculation formula is as follow.
T.P of LL (Input) = 3600 / { ATM robot place time + (ATM door close time + Pump time with glass + Slot valve open time) + VR pick time +
 (Slot valve close time + Vent time without glass + ATM door open time) }
Glass transfer order of load lock(output) is ATM robot pick from LL, ATM door close, pumping in the condition that glass does not exist in LL, slot valve open, vacuum robot placement, vent in the condition that glass exists and ATM door valve open. Its throughput calculation formula is as follow.
T.P of LL (Output) = 3600 / { ATM robot pick time + (ATM door close time + Pump time without glass + Slot valve open time) + VR place
 time + (Slot valve close time + Vent time with glass + ATM door open time) }
Glass transfer order of ATM robot is ATM robot pick from cassette, T/Z/S, placement in LL, T/Z/S, pick from LL, T/Z/S, placement in cassette and T/Z/S. For wafer system, the order of wafer alignment (placement in aligner, alignment and pick from aligner) is added. Its throughput calculation formula is as follow
T.P of LL ATM robot (Glass) = 3600 / { 2 X ( ATM robot pick time + ATM robot place time) + 4 X T/Z/S }
T.P of LL ATM robot (Wafer) = 3600 / { 2 X ( ATM robot pick time + ATM robot place time) + 4 X T/Z/S + Wafer align time }
It is possible to check the weakest step through the throughput simulation of 5 glass or wafer movement steps above. Throughput can be improved by correcting the steps.
The improvement of bottle neck in each step is as follow.
  • PM : If process time can’t be reduced, increase PM amount.
  • Vacuum robot: The method to increase the speed of vacuum robot is mainly divided into two methods. The first method is to increase the speed of robot by improving end effector shape of robot within the scope that does not cause slippage of return material. The second method is to apply the robot arm proper for used system. Dual arm is better than single arm to improve throughput. Depending on the structure of robot arm, one type has faster swap time in the single direction and the other type has faster swap time in the opposite direction.
  • Load Lock: If load lock is bottle neck, throughput can be improved by reducing volume of chamber as much as possible and improving pump spec and pipe design. In addition, pumping/venting time can be reduced by improving gas line.
  • ATM robot: The method to increase the speed of ATM robot is mainly divided into two methods. The first method is to improve grip method of end effector. The grip methods of end effector are mainly divided into edge grip, vacuum grip and passive grip. Among them, edge grip and vacuum grip have fast speeds but are weak against particle. Passive grip has relatively low speed but it can reduce the occurrence of particle significantly. The second method is to apply the robot arm proper for used system. Dual arm is better than single arm to improve throughput. Depending on the structure of dual arm, there are the method to operate two arms separately and the method to use one arm but use two wrists on top and bottom.
The table below shows the examples of wafer system. Its system structure is as follow.
System configuration
Number of PM 3
Process time (Case 1) 50
Process time (Case 2) 150
Load Lock 1 Wafer in
Load Lock 2 Wafer out
Vacuum robot Dual arm
ATM robot Dual arm
Aligner 1 ea
Software overhead 10%
ATM robot (Dual arm)
Pick/Place time Wafer align time
Place Load Lock 1 3 Pick FOUP 3
Place Load Lock 2 3 Place FOUP 3
Pick Load Lock 1 3 Rotate/Z/S 3
Pick Load Lock 2 3 Align (Place/Align/Pick) 3
 
Load Lock pump/vent
Pump time Vent time
Pump (Wafer) 15 Vent (Wafer) 15
Pump (None wafer) 7 Vent (None wafer) 7
 
Valves
Door valve time Slot valve time
Door valve on 1 Slot valve on 1
Door valve off 1 Slot valve off 1
Vacuum robot
Pick/Place Load Lock time Wafer swap time
Place Load Lock 3.5 Swap load lock 8
Place PM chamber 3.5 Swap PM chamber 8
Rotate average 1.5
Following values can be calculated through each step formula above
  • 1 (Case 1). Throughput for PM time 50sec
    3 X (3600/50) = 216 WPH
  • 1 (Case 2). Throughput for PM time 150sec
    3 X (3600/150) = 72 WPH
  • 2. Throughput of ATM robot (including aligner)
    3600/{2X(3+3)+17+4X1} = 109 WPH
  • 3. Throughput of load lock (In)
    3600/{3+(1+15+1)+3.5+(1+7+1)} = 111 WPH
  • 4. Load Lock (OUT)의 throughput
    3600/{3+(1+7+1)+3.5+(1+15+1)} = 111 WPH
  • 5. Throughput of vacuum robot
    3600/(8+8+2X1.5) = 189.5 WPH
Let’s find out the throughput of automation excluding PM through the result value collected from 1~5 Step above. On the assumption that PM time is 0, the lowest value out of 2~5 Step is equal to automation mechanical throughput. The value adding system software overhead (10%) to automation mechanical throughput is equal to automation throughput.
The formulas of automation mechanical throughput and automation throughput are as follows.
Automation mechanical throughput = Lowest WPH at Step 2~5
 Automation throughput = (Lowest WPH at Step 2~5) / (Software overhead)
Therefore, in the system, automation throughput can be calculated as follow.
Automation mechanical throughput = 109 WPH
 Automation throughput = (109) / (1+10/100) = 99 WPH
Then, let’s find out the calculation method of maximum PM time. Maximum PM time means the PM time, whose throughput does not increase any more in spite of small process time. Its formula is as follow.
Maximum PM time = number of PMs X [ 3600 / Automation throughput ? ( slot valve open + vacuum robot place to PM + slot valve close ) ]
Maximum PM time of the system = 3 X [3600/99-(1+8+1)] = 99sec
In the system, final mechanical throughput calculation formulas can be divided into following two methods.
Case 1. When PM time is less than 99 sec, it is same as automation throughput.
Case 2. When PM time is more than 99 sec,
3600 / { PM time + ( Slot valve open + Vacuum robot place to PM + slot valve close ) } X number of PMs
3600/(150+10)X3 = 54 WPH
In the system, mechanical throughput simulation results are as follows.
Case 1. When PM time is 50sec, mechanical throughput is 99 WPH
Case 2. When PM time is 150sec, mechanical throughput is 54 WPH