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RESEARCHERS CUT SOLAR CELL PRODUCTION TIME IN HALF WITHOUT LOSING EFFICIENCY

For Immediate Release
July 24, 1997

Researchers have successfully cut in half the time it takes to make a silicon solar cell without diminishing its performance, an achievement that should reduce the cost of solar energy.

Using rapid thermal processing (RTP), researchers have produced a solar cell with the same efficiency rating -- 18 percent -- as one made by conventional furnace processing. They created the cells in 8 1/2 hours, compared with the 17 hours needed for a furnace-processed cell.

In a separate process, researchers also integrated RTP with screen-printing, an alternative method for applying the cell's metal contacts, which slashed the processing time even further, to 1 1/2 hours. photo copyright information
Researchers have reduced the time required to produce solar cells without losing efficiency. (200-dpi JPEG version - 370k)

"If we can make the solar cells very fast compared to what's being done out in industry today, that will obviously reduce the use of chemicals, gases and manpower, and it will increase the production capacity and throughput," said the professor. "That can result in the significant reduction of costs."

Rapid Thermal Processing Uses Incoherent Radiation

Rapid thermal processing utilizes incoherent radiation as a source of optical and thermal energy. The interaction of high energy photons with matter leads to thermal and photophysical effects that significantly decrease the activation energy for various semiconductor processes like diffusion, thus reducing the processing time and temperature needed to fabricate a solar cell. Conventional furnace processing lacks these high energy photons and requires greater thermal cycle time and temperature.

Rapid thermal processing (RTP) provides a way to rapidly heat wafers to an elevated temperature to perform relatively short processes, typically less than 1-2 minutes long. Over the years, RTP has become essential to the manufacture of advanced semiconductors, where it is used for oxidation, annealing, silicide formation and deposition.

An RTP system heats wafers singly, using radiant energy sources controlled by a pyrometer that measures the wafer's temperature. Previous thermal processing was based on batch furnaces, where a large batch of wafers is heated in a tube. Batch furnaces are still widely used, but are more appropriate for relatively long processes of more than 10 minutes.

RTP is a flexible technology that provides fast heating and cooling to process temperatures of ~200-1300°C with ramp rates typically 20-250°C/sec, combined with excellent gas ambient control, allowing the creation of sophisticated multistage processes within one processing recipe. This capability to process at elevated temperatures for short time periods is crucial because advanced semiconductor fabrication requires thermal budget minimization to restrict dopant diffusion. Replacement of the slower batch processes with RTP also enables some device makers to greatly reduce manufacturing cycle time, an especially valuable benefit during yield ramps and where cycle-time minimization has economic value.

RTP systems use a variety of heating configurations, energy sources and temperature control methods. The most widespread approach involves heating the wafer using banks of tungsten-halogen lamps because these provide a convenient, efficient and fast-reacting thermal source that is easily controlled. In a typical RTP system , the wafer is heated by two banks of linear lamps — one above and one below it. The lamps are further subdivided into groups or zones that can be individually programmed with various powers to maximize temperature uniformity. In RTP, the energy sources face the wafer surfaces rather than heating its edge, as happens in a batch furnace. Thus, RTP systems can process large wafers without compromising process uniformity or ramp rates. RTP systems frequently incorporate the capability to rotate the wafer for better uniformity.

The AW software controller has great temperature control and repeatability, even at low temperatures for the AG610. However, operators can upset the software by starting the process at different temperatures, thus making the temperature control not so repeatable and stable under these conditions. To avoid this, a multi-step recipe to process the wafer is used, Figure 2. The first steady-state step preheats the chamber to a low temperature (for example, 150?C) to stabilize the temperature control. This is the preheat/stabilizer step. The process then ramps up to another low temperature (for example, 250?C) steady-state step and the control software checks the stability of the temperature control. This is the stability check step. The temperature is still low enough that it has no effect on the physical properties of the wafer. If the temperature control stability is within user defined limits, then the process control continues to process the wafer. However, if the stability is not within the limits, the process is aborted, thus saving the wafer. The wafer can then be re processed and, thus, there is a higher yield.

These parameter limits are called PSum1 and PSum2. For this process, set PSum1 to zero (0) and refer to the PSum subsection of “Optimizing a Recipe” of this manual to set PSum2. PSum1 looks at the first steady-state step, which is the preheat/stabilizer step. The temperature control is usually not very stable during this step, until the end. Therefore, checking PSum against PSum1 needs to be disabled by defining it to be zero (0). PSum2 looks at the second steady-state step, which is the stability check step. Here, the temperature control needs to be stable and reliable before the software will allow the process to continue.