5. Processing Hardware
Process stations are graphite crucibles
Sublimation used to deposit material
Benefits:
High material utilization
Moderate temperatures and vacuum levels
required
High quality films produced rapidly
Challenges:
Film uniformity driven by thermal uniformity
and hardware geometry
Deposition hardware costly to machine
7. Modeling Approach
Thin-Film Processing Hardware
Modeling Difficulties:
Deposition and condensation surface chemistry
Low pressure physics must be accounted for at 40mTorr
Flow at walls require special consideration
8. Sublimation and condensation are the two dominant reactions that take place
Arrhenius rate equation used by Fluent
Sc: Sticking coefficient
Calculated after experiments
A: Pre-exponential factor
Calculated for each reaction
EA: Activation energy
β: Temperature exponent
R: Universal gas constant
T: Temperature
Modeling Approach
Thin-Film Processing Hardware
9. Modeling Results
Thin-Film Processing Hardware
Cd gas molar fraction in the pocket Cd growth rate on the substrate (kg/m2s)
Vapor distribution and film uniformity can be analyzed
10. Modeling Results
Thin-Film Processing Hardware
Simulation-based engineering analysis provides otherwise unobtainable insight
Flow lines colored by Cd
molar fraction
11. Experimental Validation
Thin-Film Processing Hardware
Results validated by comparing modeled
and deposited film thicknesses
Scanning White Light Interferometry
Sticking coefficient applied from initial
experiments
12. Experimental Results
Thin-Film Processing Hardware
Modeled film thickness correlates
strongly with experimental results
Validated model used to improve new
source design before production
13. Hardware Design Improvement
Model used to predict film growth
Same equations and boundary conditions
Different geometry
Improved film uniformity
Deeper pocket
Shallower wells
Gen 1 Gen 2
1st Generation
2nd Generation
14. Hardware Redesign Results
The model predicts that the 2nd Generation source should produce more
uniform films
1st Generation
Contours of CdS film thickness: Each line represents a 1% change in thickness
2nd Generation
15. Hardware Redesign Results
Film uniformity experimentally matches predicted values
Uniformity improved by over 70% with one design iteration
1st Generation 2nd Generation
16. Continuing Work
Modeling different thin-film material evaporation processes
CdS
CdTe
CuCl
CdCl2
Deposition Rates in
(nm/s)
18. Thin Film Product Operation
New thin-film PV module design:
Designed for UV and moisture
resistance
No lamination or batching
required
Small factory footprint
Patent pending
Source: Nordson.com
19. Prototype Architecture
Two panes of custom made glass
1200 x 600 x 3.2mm each
2+ encapsulating polymers with additives
Air gap between glass panes
3μm thick semiconductor film
Top Glass
Bottom Glass
Silicone PIB Low cost polymer /
desiccant
CdTe Film
Desiccated
gap
X-section of module edge
20. Modeling Approach
Thin Film Product Operation
Over 3 million elements used
Convection boundary conditions
obtained from 2D model
Film represented as surface
Wind velocity
(m/s)
21. Modeling Approach
Thin Film Product Operation
Radiation heat transfer must be considered
Real world solar spectrum used
Unique, wavelength-based quantum efficiency of
the device accounted for
22. Prototype and industry-standard
devices modeled and compared
Numerous convection and radiation
conditions queried
Operating matrix created to analyze
thermal response trends
Modeling Results
Thin Film Product Operation
Film temperature (K)
23. Experimental Results
Thin Film Product Operation
Both devices thermal response observed
Real-world solar and wind conditions
measured at nearby station
Experimental conditions input as boundary
conditions for model
Experimental results match modeled values
24. Conclusion
Method for modeling thin-film processing demonstrated
Method can be used to improve hardware before manufacturing
Thin-film product operation in real-world conditions modeled
Simulation is valuable for thin-film processing and product design
before manufacturing
