1. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 Regular Flow Line Models for Semiconductor Cluster Tools: James R. Morrison Assistant Professor - KAIST Industrial & Systems Engineering A Case of Lot Dependent Process Times

2. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 2 Presentation Outline Motivation System description: Clustered photolithography tools & flow line model Recursions for wafer delay & extensions Computation Application to a clustered photolithography tool Concluding remarks

3. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 3 Motivation (1) Fab simulation is very commonly used in semiconductor mfg – Assess implications of changes to equipment & operations – Trade-offs between model fidelity/data collection and computation Existing fab-level simulation models – Simplified equipment representation is good for computation – Generally of adequate fidelity for most purposes – Detailed wafer robot models NOT used Industry trends: Render existing simulation models obsolete – Cluster tools have become increasingly more common – Anticipated 450 mm wafer era and/or many products Image source: http://www.semiconductor-design.com/uploads/images/wafer.jpg

4. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 4 Motivation (2) Current equipment models do NOT well address – Internal wafers buffers & state dependent setups – These are common in photolithography clusters! Need expressive yet computationally tractable equipment models of semiconductor manufacturing equipment Goals – Develop models for cluster tools (clustered photolithography) – Expressive: Incorporate internal wafer buffers & setups, transient – Tractable: Ignore wafer transport robot & appeal to system structure – Practical: High fidelity when describing actual tool behavior Image source: http://www.fabtech.org/

5. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 5 System Description: Clustered Photolithography Conceptual diagram of a clustered photolithography tool Internal wafer buffer may be present before/after the scanner Setups are of two types – Pre-scan track: Can start only after all of its modules are empty – Scanner: Setup starts once the first wafer arrives Bottleneck process Process 2: 3 modules

6. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 6 System Description: Flow Line Model (1) Modeling relaxations – Ignore wafer transport robot except for addition to process time – Process 2 is modeled as 3 modules each with 1/3 original process time – Each buffer space modeled as a server with zero process time Process times are deterministic, but wafer class dependent – t j k, for module j an d wafer class k (there are K classes) To enable the analysis, we make further assumptions – Restrictive, but as we will see, they still allow for high fidelity … Pre-scan trackBufferScannerPost-scan track Wafers Enter Wafers Exit

7. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 7 System Description: Flow Line Model (2) Let x j (w) := start time of wafer w in module m j Let aw := arrival time of wafer w to the queue Assume wafers are served in a FIFO manner (this can be relaxed easily) There are M modules in the system Wafer advancement in the flow line obeys the elementary evolution equations FS: Full Simulation

8. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 8 System Description: Flow Line Model (3) Assumption A1: Service times between wafer class m1m1 tj1tj1 m2m2 m3m3 m4m4 … m M-3 m M-2 m M-1 mMmM m1m1 tjktjk m2m2 m3m3 m4m4 … m M-3 m M-2 m M-1 mMmM t j k =  k t j 1, 0 <  k < 1

9. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 9 System Description: Flow Line Model (4) Definition: Dominating Modules. For each wafer, the modules that have strictly greater process time than all preceding modules Note: They are the same for all wafers by Assumption A1 Definition: Channel. The modules including and between any two adjacent dominating modules m1m1 tj1tj1 m2m2 m3m3 m4m4 … m M-3 m M-2 m M-1 mMmM

10. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 10 System Description: Flow Line Model (5) Assumption A2: Service times in the channels decay geometrically in each channel at constant rate  =  1 *…*  K This assumption guarantees that wafers will not experience contention unless all downstream modules are full m1m1 tj1tj1 m2m2 m3m3 m4m4 … m M-3 m M-2 m M-1 mMmM  = 1/2

11. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 11 Recursions for Wafer Delay (1) Terminology: d j (w) := delay wafer w experiences in module m j Y  (w) := total delay wafer w experiences in channel-  S  (w) := max delay wafer w can experience in channel-  x j (w) := start time of wafer w in module m j Key Result 1: Under Assumptions A1 and A2, where Y(0) = 0, a 0 = -∞, d 0 (0) = 0, d 1 (0) = 0. Further,

12. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 12 Recursions for Wafer Delay (2) The following features can be incorporated: – Wafers arrive in batches called lots (batch arrivals – wafer lots) – Track setups – Setups at the bottleneck module (scanner) Key Result 2: The equations for each channel can be strung together to give recursions for the wafer delay in the entire flow line Features of the results – Don’t have to conduct full simulation (FS) – Simply keep track of the state of each channel

13. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 13 Computational Complexity Key Result 3: Allow setups and batch arrivals of wafers – Let G be the number of lots, each with W wafers – Let B be the number of modules – Let K be the number of classes – Computations for initialization – Computations for recursions

14. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 14 Application to a Clustered Photolithography Tool (1) Let K = 20 classes of lots W = 12 wafers/lot B = 40 modules (about right for a clustered scanner with buffer) Want to simulate the system for G wafer lots FS requires approximately 960G/145G = 6.6 times more computation Computation for initializationComputation for recursion

15. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 15 Application to a Clustered Photolithography Tool (2) How good is the model when compared against data from a real tool? Throughput accurate to within 1% Cycle time accurate to within 4% Quite acceptable for use in fab level simulation

16. ©2009 – James R. Morrison – IEEE CASE 2009 – August 25, 2009 - 16 Concluding Remarks Semiconductor manufacturing environment & needs – Increase in setups, product diversity & transient behavior – Simulation is the tool of choice to assess changes at the fab level – Simulation models do not well address such features in key tools Developed a flow line model for cluster tools Computationally, the method can be more efficient than full simulation for typical clustered scanners Future work – Simplified models: Can we improve computation with minimal loss of fidelity?