SolCIS
Innovative Solutions for CIS- and Thin Film based Solar Technology and Applications
Patented* Forced Convection
Formation Process & Equipment
A new type of furnace is presented based on forced convection heating and cooling of large batches of coated glass substrates. The custom designed multiple chamber mass production equipment can handle very corrosive, inflammable and toxic processing gases from atmospheric pressure to vacuum at high durability. Flexible thermal profiling up to temperatures above the softening point of soda lime glass is precisely controlled by fast response heating units. The batches can carry more than 200 substrates each and are transferred from one chamber to the other in an indexing mode. The multiple chamber design in combination with large batches yields high production capacity even at cycle times above one hour. In this way “best of both worlds” from batch type and inline processing is exploited.
Fundamental Properties:
The equipment is designed for processing very homogenous thin films on plain substrates, especially on plain glass substrates at very high throughput in a temperature range from room temperature up to 800°C and above and at a total pressure range from vacuum to atmospheric and above.
In general the thin film(s) will be pre-coated onto the substrates by well-known deposition processes like e.g. sputter deposition, thermal evaporation or chemical vapor deposition. The thin films can be electrically insulating, conducting or semiconducting. The coating can be applied single sided or double sided onto the substrates and can represent a wide range of radiation heat transfer emissivity coefficients. It can cover the substrates completely or partially and it might be patterned, optically transparent, semitransparent or opaque. The pre-coating might also be a stack of thin films of different thicknesses and physical properties.
In principle the furnace can handle substrate sizes of 1x2m² and above, the first generation is designed for substrate sizes up to 0.6x1.2m². Substrate thickness is typically from 1mm to 3mm but can also go beyond that range.
The substrates are processed in batches in a pilot line up to 102 substrates of 3mm thickness, the substrate load per batch carrier in the current system limited to 600kg. Next generation systems will handle glass substrate loads up to 2000kg per batch.
The equipment is capable to perform processing in a wide range of processing gases from inert to reactive, inflammable, explosive, corrosive and very toxic.
The heat transfer is accomplished by forced convection. A wide range of gas velocities can be adjusted from laminar to turbulent flow.
Amongst others heating and cooling rates are dependent on temperature, total pressure, processing- and carrier gas species, gas velocity as well as substrate thickness and spacing. Typical heating- and cooling rates up to 100K/min and above can be reached depending on the substrate thickness and thermal load. Typical cycle times are in the rage of several ten minutes up to several hours.
Fundamental Design Features of the Processing Chamber:
The schematic figure below displays the fundamental features. Design and arrangement of the different features may vary depending on the special task. The processing chamber consists of a gas tight containment, e.g. a vacuum chamber made out of stainless steel. The walls of the chamber can be kept to an adjustable constant temperature in a range from room temperature to e.g. 200°C. This feature is useful in order to avoid condensation of processing gases in certain applications. At the inner surface of the chamber a thermal insulation can be applied in order to avoid heat loss and protect the chamber walls against processing gas interaction like e.g. corrosion or condensation. The chamber can be pre-conditioned in atmosphere by pump purge cycles an inert gas- or processing gas inlet. At least one ventilator per chamber is circulating the processing gases through the substrate batch and the heater matrix. The ventilator is not fixed to an axial design but can have each commonly used design depending on the specific requirements on gas flow, pressure and pressure differential. Switching from heating to cooling can be accomplished by changing the position of the flaps accordingly. Intermediate positions can be controlled and are helpful for adjustment of cooling rates in a wide range. In order to guide the circulating gas flow and to avoid direct heat radiation from the heater matrix to the substrate batch or from the substrate batch to the cooler matrix separation walls are placed between heater and substrate batch and between substrate batch and cooler. The separation walls can be covered by layers of insulation materials in order to improve thermal homogeneity on the substrates.
In the figure below a schematic cross section in plane of the production flow of the multiple- chamber forced convection system is displayed. The scheme shows the three-chamber-system, each chamber applying forced convection by ventilators. The system is fully automated and is not limited to the GEN1 three chamber version but can consist of a plurality of processing chambers, entrance- and exit load locks. The substrate batch is transported from outside the equipment into a first chamber – the entrance load lock and preheat chamber. Here the atmospheric and temperature preconditioning is accomplished like e.g. reducing residual gases like oxygen or water vapor below a specified level and heating substrate batch to a specified preheat temperature setpoint. After finalizing this step a huge valve to the processing chamber gets opened and the batch gets transported to the processing chamber. After closing the valve processing of the pre-coated substrates is started. The process control is fully automated accomplished in a wide range of recipes for processing gases, heating and cooling profiles , ventilator speed profiles, total pressure-, processing gas partial pressure- and chamber wall tempering profiles. In this way the system is very flexible in fulfilling a wide range of processing tasks for thin film applications.
The patent families listed in the link above cover the forced convection equipment as well as the process for manufacturing semiconductors - in particular for high performance photovoltaic thin film CIGS modules.
