Solar Cell Processing Equipment
With solar energy at the forefront of alternative energy initiatives around the world, companies in the solar industry need innovative, high quality equipment to keep up with increasing global demand for photovoltaic cells.
PVI Selenization Systems for Solar Cell Processing
PVI designs and manufactures Selenization systems that contain one or two horizontal process chambers fabricated from Quartz. The process chamber is a Bell-jar type cylinder with one end being formed in a domed shape. The process chambers are positioned in a system support console that houses other system components. Each process chamber has a dedicated vacuum pump, gauges, and related equipment. Multiple Selenization systems can be controlled from a common process control system.
PVI’s Selenization systems are designed and manufactured to have a high degree of reliability, safety, and flexibility. The support frame that contains the Quartz chamber has been designed to allow lifting from the bottom of the system. The system is designed to allow primary maintenance to be accomplished from one side of the system frame.
The load end of the process chamber is O-ring sealed to a stainless steel collar assembly. The collar assembly is configured with a door with an integral quartz radiation baffle assembly. Feedthroughs and fittings are provided for pressure monitoring, process gas flow, pumping, venting, and auxiliary purposes. The feedthrough ports are located in the top ¾ of the collar assembly. The door assembly is hinged and provides full access to the process chamber.
Resistance type IR heaters encompass the outer diameter of the process chamber. The heaters are mounted inside of an insulated enclosure to protect them from damage and to provide protection to the operator from high temperatures. A gas cooling/recirculation system with an integral heat exchanger and a ballast tank is provided to assist with cooling the process chamber(s).
The process control system manages all aspects of the Selenization system. Interlock protection is provided to protect the operator and system from over-temperature and other unsafe conditions. PVI provides process control software that has been developed and tested for the Selenization of glass substrate based CIGS devices.
Six heater zones encompass the outer diameter of the process chamber. The six separate heating zones are divided between the top & bottom sections of the chamber. The heaters are inside of a secondary containment enclosure to protect them from damage and to provide protection to the operators. The main heater covers the overall length of the substrate and two shorter heaters are installed on each end of the chamber in order to maintain a uniform temperature throughout the length of the hot zone.
An air gap between the ID of the heaters and the OD of the chamber is provided to allow a high flow rate of gas during cooling. Instrumentation is provided to measure blower inlet temperature and to determine that cooling water is flowing.
The main chamber door O-ring is installed on the collar assembly.
A ballast tank with high conductance plumbing is provided for the quartz chamber. The ballast tank will have a volume equal to the chamber volume and will be configured with two ports on opposite ends of the tank. An oxygen sensor is provided to control the Oxygen level in the ballast tank.
A stainless steel collar assembly that is water-cooled is provided. The collar is designed to accommodate the enclosure wall & the ring plenum for the cooling gas. The collar is supported independently from the quartz chamber and is mounted on linear slides for ease of access to the process chamber O-ring seal.
Quartz vacuum chambers up to 1000 mm in diameter and up to 2000 mm in usable length can be provided.
The control system is divided into three sub-systems: The system controller, the process controller, and the process equipment. A single system controller operates all of the process controllers through a communication network. The system controller monitors process controller operation creates and downloads process recipes and log run time data. The process controllers operate the process equipment without the help of the system controller to run the process recipe.
In this application, the system controller acts as a SCADA system (Supervisory Control and Data Acquisition). The system controller handles communications with the process controllers and acts as the operator interface and data acquisition system. A single system controller is connected to several process controllers through an Ethernet hub and network. An OPC/DDE server running in the background will link variables in the system controller to values in the PLC process controllers. Through this communication link, the system controller is easily located away from the process controllers and equipment.
The system controller consists of a PC using the Microsoft Windows operating system running an application written in National Instruments LabWindows/CVI. The graphical user interface gives the operator control of the process. Temperatures, pressures, flows and equipment status will be displayed on several screens organized by function.
Password protected operating levels provide security by logging system operators and prevent unauthorized changes in the system configuration. Operator level will allow the selection of process recipes, running the process, monitoring the process and acquiring data. Engineer access will have all operator privileges, have access to configuration and process editing screens and manual control of all system components.
The process controllers are monitored through a series of display screens that show the current status of each process chamber. The top-level display will show all of the process controllers and a summary of their current condition. Each system can be highlighted in a color that at a glance will show if the unit is in-process, faulted or waiting to be loaded or unloaded. Current temperature, pressure, process step and process time is included.
By clicking on one of the systems on the display, another more detailed screen can be shown that contains more detailed process information. This screen can control the operation of the selected system. The process can be started or aborted, new recipes can be downloaded and temperature, pressure and flow graphs can be displayed. Engineering level access will allow modification of that system's constant, creation, and modification of new recipes and manual operation of system components.
Configuration screens contain all system-related operating constants. Constants include vacuum crossover pressures, maximum operating pressures, cooling parameters, and alarm limits.
The process recipe screens which contain all information related to running the process sequence can be edited and saved by an engineer for later retrieval by the operator prior to running the system. Multiple recipes can be saved for each process type. The selection of a recipe uses standard windows file dialog boxes.
The process controller is PLC based and uses industrial rated digital and analog I/O. The process controller directly operates all system components. Automatic sequences programmed inside the process controller provide pressure control, temperature profiles and gas flow control to follow a downloaded process recipe. All functions are interlocked to prevent unsafe operation or damage to the equipment.
Digital I/O monitors and controls the operation of pumps, valves, contactors, and limit switches. Analog I/O monitors and control flow rates, pressures and temperatures. Equipment status and process values are also monitored by the system controller.
Temperature control is performed inside the PLC to allow synchronization of all six temperature control zones. A downloaded recipe describes the temperature profile required by the process. Each zone has a separate PID control loop with individual tuning constants.
Two buttons are provided near each furnace to remotely run the process. One button will start the currently loaded process recipe and the other is used to vent the system when the process is finished. The buttons are illuminated to show the current running status of the system.
Control system integrity is greatly improved by separating the system controller from the process controller. Once started, the process controller will complete the process recipe even if the system controller fails or is disconnected. The system controller performs the non-real time functions for monitoring, configuration, and supervisory control. The process controller reacts in real-time to complete the process recipe downloaded by the SCADA system. This greatly improves response time to process requirements.
The process equipment is the actual hardware used to operate the vacuum system. Pumps, valves, flow controllers, pressure sensors, temperature sensors, and motors are all monitored and operated by the process controller.
The vacuum system consists of a heated quartz vacuum chamber pumped by a rotary vane vacuum pump through two flow paths. The first flow path contains a filter to block debris from entering the vacuum pump and a throttle valve to control the pump-down rate. The second flow path is used as a high conductance path to reduce the vacuum chamber pressure to a low level after initial pumping. The pressure in the vacuum chamber is monitored by two capacitance manometers, one for low pressures and one for high pressures. The vacuum pump inlet and outlet pressure are monitored to detect any pump or exhaust problems.
Gas Flow System
The flow system controls gas flows into the vacuum chamber. Gas flow into the system uses one of two paths, a bypass path to the pump for flow stabilization or gas line evacuation and an inlet path to the chamber. The bypass and inlet valves are part of the Selenization system. Process gas flow rates are controlled by the customer's mass flow controllers and connected to the Selenization system through an interface panel. Analog I/O modules in the Selenization system process controller will set and monitor flow rates.
Temperature Control System
The temperature control system consists of six hot zones. Each zone control loop consists of a thermocouple, thermocouple input point, PLC PID loop, an output point, an SCR power control module and a heating element.
Dual K-type thermocouples are used to monitor the temperatures near the surface of the quartz cylinder near the center of each heating zone. One thermocouple signal is used for temperature control and the second is used as a backup to prevent damage to the system in the event of a sensor or control failure.
A thermocouple input module in the PLC converts the temperature signals into usable numeric values inside the PLC. A PID loop inside the PLC computes an output value based on the difference between the current temperature and the setpoint value from the recipe profile. The output value then controls an SSR power module to deliver a precise amount of electrical power to the heating elements to maintain the required temperature.
The gas recirculation system is controlled by the PLC through a variable-speed blower. The blower speed during the cooldown is set in the configuration screen. The blower inlet temperature is monitored to protect the blower. The Oxygen level in the recirculation gas can be controlled to a level set in the configuration screen.
Power Distribution and Control
The main power inlet is a fused disconnect switch which will both isolate the equipment from electrical power for servicing and protect the system and internal circuit breakers from damage in the event of a catastrophic short circuit. AC power distribution inside the system is through circuit breakers to allow easy resetting in the event of an accidental short or overload.
A 24 volt DC power supply provided control power to the system. This low voltage control improves the safety of the system and is power limited in the event of an overload. An emergency stop and master control relay circuit is used to remove control power from the system in the event of an emergency. Terminals are provided to allow the connection of additional emergency stop buttons or signals and contact is available for monitoring the status of the emergency stop circuit. A +/-15 volt power supply provides power to the mass flow controllers and the capacitance manometers.
Power to the vacuum pump is through a motor starter and overload circuit. This circuit is used to start and stop the pump and protect it from overloads. Contactors are used to isolate the heater circuits from the power source when the heaters are turned off.
All valves in the system except the throttle valve are pneumatically powered through a valve bank which is supplied from a pressure regulator. Pneumatic power can be either CDA or nitrogen.