The following are a series of frequently asked questions about the Natcore liquid phase deposition (LPD) process. We’ve found that the answers to these questions help to explain what the Natcore process is, and what it is not.

Q. Is this a sol-gel process?
A. No, this is not a sol-gel process. Natcore’s process is a development of a process called liquid phase deposition (LPD).

Q. How does Natcore’s process differ from a sol-gel process?
A. The sol-gel process requires the reaction of an organic-containing precursor that results in the incorporation of impurities into the sol-gel formed material. These impurities must be removed by pyrolysis. In contrast, Natcore’s process uses no organic compounds and, therefore, the associated impurities cannot be incorporated.

Q. Has LPD been used before?
A. Yes, successfully by a number of research groups. However, unlike prior research in this area, Natcore’s patent-applied for and proprietary technology allows for faster growth rates to be maintained over a long reaction time.

Q. Is purity of the film the only difference between sol-gel and Natcore’s process?
A. No, the sol-gel process results in homogeneous nucleation, causing solids to form within the solution instead of on the desired surface. These particles, formed in solution, then drop onto the surface being coated, causing defects. In contrast, Natcore’s process is shown to react by heterogeneous nucleation at the interface between the growth surface and the reagent solution. Therefore, Natcore’s process grows a film at the surface, where the film is required.

Q. Is this is a slow process?
A. Traditional liquid methods give fast film growth initially, but the rate of growth then slows. In contrast, Natcore’s technology allow for faster growth rates to be maintained over a long reaction time.

Q. What are the deposition temperature and deposition rate?
The deposition temperature is around 40°C. At room temperature (20°C), the deposition rate is 20 nm/hr. At 30°C, it's 40 nm/hr. At 50°C, it's 60-70 nm/hr.

Q. What is the basic process flow of the LPD coating?
A. The process flow for silicon wafer-based products is: Wafer cleaning---Wafer coating---Wafer rinsing---Wafer drying. Film growth proceeds at room temperature or at slightly elevated temperatures (up to 40°C). The oxide thickness can be selected to be anything between a few nanometers to a few microns.

Q. Is there an additional annealing process after LPD?
A. There is no annealing step after the LPD process for silicon dioxide.

Q. Is the wafer-cleaning step before the LPD compatible with the phosphorus diffusion step that precedes the anti-reflective coating deposition?
We use several different standard wafer-cleaning processes prior to growing the LPD silicon dioxide films. We typically use HF/ozone cleans (fluorozone) and modified SC1/SC2 processes with similar results. We would be happy to test other specific cleans if a customer uses an alternative process.

Q. Is Natcore's process limited to coating silicon wafers?
A. No, part of Natcore's process is the ability to deposit a coating on a range of metal substrates. Among potential applications, this enables is the formation of circuits of optical interconnects on computer circuit boards using their present metal interconnects as guides.

Q. Can Natcore grow materials other than silicon dioxide?
A. Yes. Natcore has grown a range of materials, and we are tailoring the thin film to the needs of the end use.

Q. What is the coating material of antireflective layers?
A. The AR coating can be silicon dioxide, silicon oxynitride or titanium oxide among other materials.

Q. Natcore Technology has achieved great results in depositing a single layer AR coating. Could this technology deposit double layer AR coating to increase the index of refraction?
A. Yes. Multiple layer coatings with silicon dioxide and titanium oxide can be made, as well as many other combinations.

Q. Presently, the popular anti-reflective coating in plasma enhanced chemical vapor deposition (PECVD) uses silicon nitride. How does that method compare with your LPD process using silicon dioxide with regard to conversion efficiency, reflectance, cost, thickness, uniformity, etc?
A. The LPD process can also incorporate nitrogen in the film and can reproduce the refractive index of silicon nitride, giving the same performance. However, the operating and equipment costs would be greatly reduced. With regard to thickness, the thickness depends on the wavelength range desired, just as in the case for the thermal and PECVD oxides. 160 nm gives an average broadband reflectance of as low as 8% on silicon wafers for wavelengths from about 1100 nm to about 500 nm.

Q. How much could the final conversion efficiency of the silicon solar cell reach, c-Si and poly-Si, respectively, using the LPD process?
A. The LPD process will achieve the same output gain for the cells as would be expected for a PECVD AR coating using the same material. The final efficiency will depend on the initial efficiency without the AR coating.

Q. Multi-crystalline wafers contain grains of various crystalline orientations. Would this impact LPD-grown film in any way?
Based on depositions run to date on multicrystalline wafers, the deposition rate is similar, and the underlying crystal morphology is translated through to the surface of the films, perhaps slightly smoother, under typical deposition conditions.

Q. What is a quantum dot?
Silicon nanocrystals, or quantum dots, are very small clusters of silicon atoms that are very hard to image even with today's high power electron microscopes. Quantum dots typically have diameters on the order of 2 nanometers to perhaps 30 nanometers. The distance between atoms in a silicon crystal is 0.235 nanometers and the diameter of a silicon atom is about the same, so a 2.5 nanometer diameter silicon quantum dot will be a little more than about 10 atoms wide.

Q. How can you measure anything so small?
The atomic force microscope (AFM) allows researchers a way to measure the diameter of quantum dots without the need for an actual visual image. Basically the AFM is a super-sensitive mechanical device that holds a stylus with a tip radius on the order of a few atoms wide itself. The stylus is suspended over the array of quantum dots that have been deposited on a substrate of some sort and is then dragged along over the surface. The stylus will trace the outline of the quantum dots as it moves over each one, and produces an image.

Q. How are quantum dots formed on the film? Is it by laser or some other process?
A. Quantum dots are preformed and introduced into the film as it grows.

Q. What are the base substrates?
A. Current substrates are silicon and quartz. Other suitably functionalized substrates are possible, such as copper, indium tin oxide (ITO) films on glass or other materials, polycarbonates and other plastics. The LPD process can be applied to any surface that can be properly functionalized so that the film bonds to the surface.

Q. What is the maximum substrate size that can be achieved with the LPD process?
A. The substrate size is not limited. The process will work on any wafer size available (currently up to 12 inches in diameter). In fact, it can be made to work on large flat sheets of square meter size or greater. It can even be made to work in a roll-to-roll manufacturing line.

Q. Is the technology protected by patents?
A. Yes, Natcore has an exclusive license from Rice University for the technology.

Q. What degree of coverage does the patent have?
A. The patent has been granted in Russia and is pending in the U.S., Europe and other countries. Natcore has performed extensive patent and literature searches and believes that our patents will be issued.

Q. Are there any applications not covered by our agreement with Rice?
A. Natcore has certain proprietary technology that it intends to file in the U.S., Europe and other countries. At present this proprietary technology has either been filed as provisional patent applications or is being kept as trade secrets. Natcore exclusively owns the rights to this additional technology.

Q. What are your competitive advantages over other processes?
A. The technical advantages of Natcore's process in comparison with other solution methods (such as sol-gel) are higher purity, greater film uniformity and deposition rate. The advantages of Natcore's process in comparison with vapor methods (such as high temperature oxidation) include the use of a lower-energy process, the use of well-established methods, and the use of low-cost equipment that is typical of a semiconductor device fabrication facility.

Q. Does that relate to a commercial advantage?
A. Yes, Natcore believes that our initial competitive advantage will be sharply reduced costs and dramatically increased throughput for oxide film growth.

Q. Current state of the art argues that greater heat and more extreme pressure differentials yield progressively better oxide films. How can you make such a claim that flies in the face of all previous experience?
A. Yes, this process is opposed to current high-energy consumption methods. And that's precisely why it is such a compelling technology, and has such important implications going forward. Natcore has taken an approach similar to nature. Man-made materials (oxide formed by high temperature oxidation) are formed under extreme conditions. In contrast, nature uses low-energy-designed pathways to make materials with high purity.

Q. So it's a new approach, but isn't it just a curiosity from a professor's laboratory?
A. While Natcore's process was originally developed in Professor Barron's laboratory within the Richard E. Smalley Institute for Nanoscale Science and Technology at Rice University, Natcore has put the process through rigorous, independent testing by a globally respected laboratory.

Q. What are the results of these independent tests?
A. The independent laboratory, Battelle Memorial Institute, was able to scale up the process as developed by Professor Barron and uniformly coat multiple wafers in a single reaction bath. They were able to quickly replicate Professor Barron's results and improve the scale and film purity. The films have passed all tests with flying colors, including analyses of uniformity, growth rates, conductivity, reproducibility and early-stage testing for purity.

Q. Have additional tests been performed?
A. Yes, two additional independent labs have been used for confirmation of the film purity.

Q. Have you compared the passivation benefit of your anti-reflective coating layer with, say, thermal oxide, PECVD-grown nitride, etc.?
A. We have compared our LPD-grown films to thermal oxides, ALD oxides, and PECVD-nitride. Based on the current process, we have obtained passivation comparable or even superior to values found in literature.

Q. Will this need a large financial commitment for implementation?
A. No, the chemicals used in Natcore's process are all typically found in a fabrication facility and are readily purchased in high purity. The equipment that will be used for Natcore's process is the same as that used in all fabrication facilities in the world for cleaning and etching silicon wafers.

Q. How much would it cost to produce this device including all the parts for the bath? How many sets are required to build a 25MW solar cell standard production line?
A. Assuming three watts per cell, eight million wafers have to be processed a year. If it is a single eight-hour shift for five days a week, the rate has to be 4,000 wafers per hour. Eight units capable of processing 500 wafers per hour would be needed.

Q What is the cost of the additive per six-inch wafer (minus the labor cost and equipment depreciation)?
A. The cost will essentially be the cost of consumable materials, in this case a small amount of fluorine and silicon dioxide. The total volume of the film on a six-inch wafer is about 0.0001 cubic cm. The cost of material is essentially negligible and the cost of the equipment will be amortized over millions of wafers in a year.

Q. What are the equipment requirements for the LPD process?
A. The basic equipment is a Teflon tub that can hold several hundred silicon wafers at one time. The liquid supply plumbing and controls would be specialized for the process.

Q. What are the carrier lifetime and the reflectance after LPD oxide passivation?
A. We are in the process of making minority carrier lifetime and surface recombination velocity measurements for silicon wafers with the LPD film on them. We expect to prove that the surface is passivated with the LPD oxide as well as it is with the thermal or CVD oxide. We will share the data as they become available.

Q. How easily can the transition from lab testing to actual production in a commercial environment be made?
A. Mass production of coated wafers will be easier and less costly with LPD than for PECVD and the equipment will occupy a much smaller footprint. A 25 cm wide by 100 cm long by 25 cm deep tub will process a minimum of 100 eight-inch diameter wafers at one time. The controls and plumbing will add to the footprint, but it is still much smaller than a PECVD system (by as much as a factor of 10).

Q. Could this LPD technology be applied to other industries besides silicon solar cells?
A. This LPD process has been used to make waveguides and could have many applications in a wide range of industries. See Other Applications.