LINEAR SOURCE TECHNOLOGY REVIEW FOR CIGS CO-EVAPORATION
ON GLASS AND FLEXIBLE SUBSTRATES
Ken Pfeiffer1, Chad Conroy2, Jian-gang Weng2, Doug Brown1, Ramya
Chandrakaran1, David Fobare3, Dave Metacarpa3, John Patrin2
1Veeco Solar Equipment Inc., 70 Olde Canal Drive, Lowell, MA 01851
2Veeco Instruments Inc., 4900 Constellation Drive, St. Paul, MN 55127
3Veeco Solar Process Development Center, 13 Corporate Drive, Clifton Park, NY12065
ABSTRACT
The highest efficiency copper, indium, gallium and
lenium (CIGS) devices have been produced using
thermal deposition methods on a variety of substrate
materials including glass and stainless steel foils. There
are veral advantages of thermal deposition; process
孤岂欲卿治经为博士邪翻译flexibility, inexpensive raw materials and high average
efficiency. The disadvantages of thermal deposition have
been low deposition rates, control complexity and the lack
of commercially available large area deposition systems.
Pursuit of the thermal deposition method has been difficult
战色with multiple point Knudn Cells (sources) due to the
costs, source-to-source variability, flux rate stability over
time, and the number of simultaneous control loops
required for feedback monitoring and control. Veeco’s
火星人中文译德文
linear metal source technology answers the challenges
by creating an array of adjustable, distributed point
sources from a single, temperature-controlled crucible for
each element integrated into one heated body which can
evaporate up to three different elements. This technology
has been scaled for applications requiring 0.35 meter (m),
0.6 m, 1.0 m, and 1.2 m deposition widths. The primary
focus of this paper will be the metal linear source
clubfootoperating conditions and performance results bad on
lab and production system deposition testing in glass and
roll-to-roll (web) stainless steel CIGS production systems
investment casting
manufactured by Veeco Solar Equipment.
METAL SOURCE DESCRIPTION
The metal linear source is typically ud for large area co-
evaporation of materials such as copper (Cu), indium (In)
and gallium (Ga). The patent-pending high temperature
design of this evaporation source simultaneously
evaporates materials contained in three heated crucibles
onto moving substrates in a top-down or bottom-up
evaporation configuration. Each of the three
5 liter (L) crucibles contained within a source has
independent temperature control, which determines each
material’s deposition flux rate. The uful volume of the
crucible, elemental density, material packing factor and
deposition rate all combine to determine the refill interval
for a linear metal source.
Element Density
(g/cc) Fill (%)
Empty
(%)
Packing
Factor
Material
(kg)
Copper 8.96 80% 20% 0.70 18.8 Gallium 5.91 80% 20% 1 17.7 Indium 7.31 80% 20% 1 21.9 Table 1 Material quantities for 5 L crucibles.
For example, a 15 megawatt (MW)/year line at 10% photoconversion efficiency requires an output Cu flux rate on the order of 40 grams/hour(g/hr) per source with three sources operating in parallel. A maximum fill level of 80% allows for a 4 L total fill volume while leaving 20%, or 1 L, of material in the crucible to ensure the most consistent flux rate throughout the deposition process. Therefore, a
single Cu-filled crucible will be able to operate for 470 hrs, or nearly 20 continuous days, between refill intervals at a 40 g/hr deposition rate. Low material utilization will decrea this refill interval, causing higher downtime, and increasing raw material costs through waste.
The crucible heating system is the primary driver of the material deposition rate, but the nozzle array determines the efficacy of vapor delivery to the substrate surface. The nozzle array is heated to high temperatures (~1550o C) to prevent any unwanted material condensation, or spitting effects, during the deposition process and ensures the full range of possible deposition rates are available to the ur. The nozzle array acts like a prearranged array of point source evaporators for each material, but with much simplified controls. The nozzles are configurable to adjust flux patterns to achieve maximum deposition uniformity by substrate width, varying source-to-substrate distance, and desired flux rates. The pattern of nozzles within the array can also be arranged to enhance material utilization of each element and has been described elwhere [1,2]. Figure 1 Metal linear source on maintenance track. The metal linear source is mounted on a frame which allows for ea of rvice access into the water-cooling
jacket which surrounds the crucible. The entire asmbly is mounted on a maintenance track to ea source removal for rapid exchange of chamber shields during routine cleaning of deposition ch
ambers.
OPERATING CONDITIONS
The four temperature controlled ctions of each linear source rely on a custom designed power supply which allows each control circuit to operate under varying input power conditions and still perform evaporation with very similar output control conditions. The controlled outputs are switched DC voltage controlled circuits operating from the condary of a Delta-Wye isolation transformer. The power supply is designed for nominal inputs ranging from 380-500 vacuum (VAC) and 50-60 Hertz (Hz) which makes it uable in Europe, Asia, and North America during (+/-10%) hi-line and low-line conditions. Typical power ratings for each controlled circuit during Cu/In/Ga metal evaporation are shown in Table 2.
Section Temperature Power (kW)
Nozzle Array 1550o C
感悟青春
40.5 Copper (Cu) 1460o C 9.1 Gallium (Ga) 1250o
C 1.0 Indium (In)
1170o C
1.1
When operating under the power conditions, the source is hot enough to emit an orange-white light which allows the ur to e into the vacuum deposition environment. Each linear source allows in power and thermocouple feedback.
Figure 2 Metal linear source in operation.
FLUX RATES AND UTILIZATION
Controllable flux rates of each element range from <1-40+ grams/hour on 0.35 m to 1.2 m width deposition zones. The proprietary design of the distribution nozzle allows for clo proximity deposition between source and substrate which in-turn increa material utilization in
excess of 65%. Cu tends to be the limiting factor for deposition rates as it has the lowest vapor pressure of the three CIGS metals (Cu/In/Ga) while having the highest atomic percent composition in the film. Therefore, the rate control of Cu is the throughput limiting factor in CIGS manufacturing process. The Veeco production metal linear sources were flux rate calibrated with real-time power monitoring and Type-C thermocouples. While power was controlled and temperature monitored, deposition rates were measured along the nozzle array centerline via water-cooled quartz crystal monitors (QCMs) as well as with coupons placed above the source.
The coated coupons were subjected to varying time intervals of steady state flux, during which the shutter mechanism was opened and subquently clod. The accumulated metal thickness was measured using a Veeco Dektak thin film profilometer. The deposition rate values were cross-checked by weighing each element’s crucible load (kg) pre- and post-deposition. As shown in Figure 3, the deposition range of interest for production throughput of Cu is 5-50 g/hr which corresponds to 1400-1450o
C crucible operating temperatures.油泵工作原理
Static off-axis measurements were also taken to validate computer models for in-motion flux profiles required to predict deposition uniformity for moving substrates.
UNIFORMITY
The design specification within each deposition zone is to achieve a cross-substrate uniformity of +/-5%, within the edge exclusion window. This level of uniformity is required for adequate CIGS compositional control and ensures matching Voc, open-circuit voltage, and Jsc, short-circuit current density, solar cell characteristics. Three steps are required in order to accomplish in-system uniformity mapping of the linear metal source. First,
proprietary computer models are ud to estimate flux rates and patterns along the nozzle array at given deposition rates. Primary causal effects such as number of nozzles, nozzle geometry, vapor pressure drop in the ambient vacuum pressure environment are included in the model. Nozzle positioning and geometry are modified to optimize the uniformity and material utilization. Second, the actual nozzle geometry and positioning is tested in a static test condition with test coupons, as shown in Figure 4. The test coupons are placed over the centerline of the nozzle array at typical s
ource-to-substrate distances. Figure 4 Copper film test coupons, static testing.
The test coupons for this test are the same ud in the deposition rate tests, yet are spaced to provide a uniformity map across the entire deposition zone. The data from the static coupon testing, as shown in Figure 5, provides the final information to indicate adjustments required to put a source into system operation for deposition with a moving substrate.
Figure 5 Measured static uniformity results.
The third and final step in uniformity testing is the scanning average test, where substrates are moved through the vapor field at production velocities and the uniformity is measured under production substrate temperatures ranging from 500-575o C at velocities ranging from (0.2-1.0) meters/minute (m/min).
When testing is conducted on 0.6m to 1.0m wide flexible substrates, the Veeco FastFlex TM Web Coating System, as shown in Figure 6, is ud. This system has three metal linear sources and three Se sources for producing CIGS on flexible substrates under a wide variety of operating conditions including:
1. Varying tensions
2. Substrate temperatures
3. Web speed
4. Vacuum levels
chink in the armor5. Deposition flux rates
Figure 6 Veeco FastFlex Web Coating CIGS System. Uniformity testing on glass substrates is conducted on the FastFlex Glass Coating System, as shown in Figure 7.
Figure 7 Veeco FastLine TM Glass Coating CIGS System.
Under typical operating conditions, material sticking coefficients, shield designs, aperture effects and other system design elements can contribute to source uniformity. Sample uniformity data from the FastFlex system are shown in Figure 8.
The tests were taken at a variety of different ttings of linear source power and web velocity on 0.6 m wide, 150 micron thick, 430 stainless steel, coated at 550o C. Excluding edge effects, the cross-web, in-system uniformity is approximately +/- 1%.
STABILITY
Due to the large sizes of the crucibles and other design elements within the linear metal source, maintaining a stable evaporation flux rate over a period of veral weeks is a significant thermal design challenge. The large thermal mass of a large source and crucible is an advantage in a production environment. Once the source is at a specified temperature a control loop algorithm can maintain a constant temperature or power to maintain a stable flux. The flux and temperature over a 56 hour time period is highlighted in Figure 9. The Cu deposition rate was 25g/hr +/-1.4%.
Figure 9 Graph showing the flux stability (green line) and the temperature stability (blue line) for the linear source.
SUMMARY
In summary, the Veeco Metal Linear Source is a family of co-evaporation enabling system compone
nts ranging from 0.35m to 1.2m deposition widths which have veral key design and performance advancements to enable scale-up of single or multi-stage co-evaporation bad thin-film CIGS manufacturing. This source is designed for extremely high temperature, continuous operation which currently enables a Cu metal-limited flux rate of 40g/hr/source. At the flux rates, the sources can operate for at least (2) weeks between material refill intervals keeping system uptime high. As line design requirements for scale-up increa, the number of sources ud as deposition modules may be incread by rial operation. This may be advantageous for the development of graded bandgap structures to increa average output efficiency. The configurable nozzle array enables 'calibrated design' of the flux patterns to achieve a high uniformity on static and most importantly, moving substrates. The ability to tune flux in a particular system environment is important to the source operation as vapor flow dynamics and ultimately the film uniformity can change with length and scaling factors inherent in the 0.35m to 1.2m deposition widths. Finally, the stability of a metal linear source determines the amount of monitoring and feedback controls required to maintain a process within a stable process window. The thermal inertia of the large crucible mass within the source allow for extremely stable flux rates over veral days without any adjustment.
REFERENCES
[1] J. Patrin, R. Bresnahan, D. Miller, “Thin Film Deposition System Optimization Using Flux Profile Modeling”, Proceedings of the 23rd European Photovoltaic Solar Energy Conference, 2008, pp. 2607-2609.
[2] J. Patrin, R. Bresnahan, T. Lampros, “Development of Thin Film Systems for CIGS on Glass and Flexible Substrates at Veeco Instruments using Linear Evaporation Sources”, Proceedings of the 24th European Photovoltaic Solar Energy Conference, 2009, pp. 2469-2472.
stimulated
