Solar cell process temperature measurements
Solar cell process temperature measurements by Fred Dimock, BTU International, North Billerica, Massachusetts, USA
Process engineers and production people continually search for better ways to establish meaningful process control systems. Some of the areas that are getting increased attention in photovoltaics are the thermal profiles required for diffusion, anti-reflective coating, and metallization in continuous or belt furnaces. Fortunately the electronics industry has established robust profiling techniques that can be applied to the diffusion and antireflective coating processes, but metallization or rapid thermal processing (RTP) remains a challenge. It is the intent of this paper to communicate better techniques for the former and show a robust method for measuring the latter.
Practical temperature measurement begins with an understanding of the available equipment, and finishes with understanding the results. Between the two end points, consistency of technique and attention to detail is paramount when accurately obtaining profile data. Profiling the diffusion process
The solar silicon cell diffusion thermal process parameters (temperature, time and uniformity) closely follow those of the thick film and low temperature co-fired ceramic industries. The standard electronic thick film process consists of screen printing metal pastes that have conductive, resistive or dielectric properties on a thin ceramic plate and then firing at 850˚C with a hold of 10 minutes in air. The issues they face are adhesion of the fired material to the ceramic and final resistive or dielectric properties of the metals. Although adhesion is universal for all three categories of paste, the electrical properties are dependant upon the peak temperature, hold time, and ramp rates during firing. It is common to have the peak temperature
specified as ± 3˚C and time at peak as ± 0.5 minutes. Frequently the time over an intermediate temperature is used to define the ramp rates and the amount of thermal energy (heat/work) the product is exposed to. When precision resistors are required, laser trimming is utilized after firing to fine tune them. Very tight control of the starting materials, screen printing process and firing have proven to minimize and at times eliminate the need for laser trimming. The equipment used for thermal profiling consists of a data logger, trailing thermocouples, and a target. Data loggers can be built into the furnace operational software or be separate systems from companies such as KIC Thermal Systems, Datapaq, or ECD. Robust 20 gage trailing thermocouples, long enough to extend completely through the furnace are used because clearances in the furnace and the time at temperature exclude the use of thermal barriers. Numerous thermocouple (TC) designs such as solid inconel sheath, ceramic fiber and inconel over braid have been used. Each one has its advantages and
Keywords: Process Control, Rapid Thermal Processing, Metallization, Temperature Measurement
Figure 1. Three TCs and targets on diffusion furnace belt.
6 – Global Solar Technology – January 2010
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