Light Enhanced Hydrofluoric Acid Passivation: A Sensitive Technique for Detecting Bulk Silicon Defects

The overall goal of this procedure is to use a room temperature surface passivation method in order to inhibit surface recombination, such that bulk silicon defects can be accurately characterized. This method can help us understand what defects will limit the lifetime of high-purity silicon wafers, which are required for very high efficiency solar cells. The main advantage of this technique is that it can be performed at room temperature, meaning the recombination activity of bulk silicon defects will not change during the measurement.

The implications of this technique extend towards measuring the recombination activity of bulk silicon defects that exist in low concentrations which otherwise would be very difficult to measure by other techniques. Generally, individuals new to this method will struggle because a requirement for high purity DI water and chemical cleaning procedures is not initially understood, thus it’s imperative the procedure is followed step-by-step. Solution preparation, including vital notes on hydrofluoric acid preparation, as well as general setup and equipment calibration instructions are all provided in the text protocol.

With those objectives achieved, proceed with cleaning the silicon wafers using chemical treatments. Begin with loading the samples into a quartz cradle. Then, transfer the cradle into a hydrofluoric acid bath meant for general use.

After about 10 seconds, the samples will be hydrophobic. Remove them from the bath, and rinse them off by passing them through three deionized water baths. Next, in a fume hood, slowly immerse the cradle in the SC1 solution, which has been maintained at a temperature of 75 degrees Celsius.

Let the treatment go for 10 minutes. And meanwhile, fill three chemically cleaned two-liter beakers with deionized water. After the SC1 treatment, rinse the samples off by passing them through the three water baths.

Next, dip the samples into a dedicated hydrofluoric acid bath for about 10 seconds. Then rinse the samples off using three fresh deionized water baths. Now, move the samples to the fume hood where the SC2 solution was prepared, and heated to a stable 75 degrees Celsius.

Immerse the samples in the SC2 solution for 10 minutes. After the SC2 treatment, rinse off the samples using the three deionized water baths. If needed, let the samples stay in the last water bath overnight.

After the rinse, dip the samples in another dedicated hydrofluoric acid bath for 10 seconds. Again, rinse the samples by passing them through three fresh deionized water baths. Now, take the samples to the fume hood where the TMAH solution is stably heated to about 85 degrees Celsius, and slowly immerse the samples into the TMAH solution to etch the wafers.

Let them react for five minutes to remove about five microns of silicon. Next, rinse off the tacky TMAH solution using at least three deionized water baths. The same rinse beakers can be re-employed here.

Once rinsed, proceed with taking measurements within two hours. This process is conducted under a fume hood. In preparation, fill two two-liter plastic beakers with deionized water.

Also prepare some plastic tweezers in the hood to handle the samples. On the computer, open the Lifetime Tester file, which contains the correct calibration coefficients for the hydrofluoric acid measurement setup. Select transient from the mode options, and enter a description of the wafer’s variables.

Now, carefully secure the lid on the hydrofluoric acid container, and move it onto the Lifetime Tester stage, centered over the blue circle, which represents the position of the inductive coil. Let the solution settle for about a minute before proceeding. Then, on the computer, click the Zero Instrument button to measure the voltage of the solution.

After taking the measurement, carefully return the hydrofluoric acid container to the fume hood bench. There, remove the lid, and rinse off any condensation on the lid with DI water from the fume hood’s water tap. Now, using tweezers, transfer one wafer into the hydrofluoric acid bath.

Lightly press the wafer down to the bottom of the bath. Then, replace the lid and carefully carry the container back to the Lifetime Tester stage. Center the wafer over the inductive coil.

Before taking a measurement, make sure the fume hood lights are switched off. Now, switch on the halogen lamp to illuminate the silicon wafer, and keep it on for about a minute. During the illumination period, in the software, click the Measure button.

The software will then begin taking data. Fill in the prompt with a file name, and set the Sample Averaging option to 10. After about a minute of illumination, switch the lamp off, and immediately click the Average button on the Taking Data window.

The software will perform 10 measurements. Then, click OK.Now, carefully return the container to the fume hood bench, cautiously remove the lid, and carefully remove the wafer. Rinse the tested wafer off in the dedicated rinse beakers, followed by a final rinse using the DI water tap in the fume hood.

Then, store the tested wafer, and repeat the process for each silicon wafer to be measured. After the experiment, follow the clean up instructions provided in the text protocol. Following the described protocol, whereby silicon wafers were treated, etched, immersed in hydrofluoric acid, and measured using the photoconductance tool, a lifetime curve, which is limited by surface recombination results, as seen by the blue triangles.

When the silicon wafer immersed in the hydrofluoric acid bath is illuminated by a halogen lamp for one minute, and then a photoconductance measurement is performed directly after illumination, a significant increase in the lifetime will occur, as seen by the red circles. This increase in lifetime is due to a reduction in surface recombination. The surface passivation began to degrade within seconds of turning off the halogen lamp.

The blue circles show the consequence of waiting for one minute post-illumination. After watching this video, you should have a good understanding of how to measure the bulk lifetime of silicon wafers using a hydrofluoric acid bath, specifically how to treat the wafers before the measurement, how to activate the surface passivation by using a light source, and how to measure the lifetime immediately after illumination. Once mastered, this technique can be performed in two to three minutes per wafer.

However, it will take an hour to prepare the wafers for the measurement, and thus it’s time efficient to measure batches of wafers. While attempting this procedure, it’s important to remember that the silicon wafers must be kept clean in order to consistently achieve very good surface passivation, and thus be able to detect and characterize bulk defects. Don’t forget that working with hydrofluoric acid can be extremely hazardous, and precautions such as wearing personal protection equipment, and following procedure step-by-step should always be taken while performing this technique.