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

Summary

A RT liquid surface passivation technique to investigate the recombination activity of bulk silicon defects is described. For the technique to be successful, three critical steps are required: (i) chemical cleaning and etching of silicon, (ii) immersion of silicon in 15% hydrofluoric acid and (iii) illumination for 1 min.

Abstract

A procedure to measure the bulk lifetime (>100 µsec) of silicon wafers by temporarily attaining a very high level of surface passivation when immersing the wafers in hydrofluoric acid (HF) is presented. By this procedure three critical steps are required to attain the bulk lifetime. Firstly, prior to immersing silicon wafers into HF, they are chemically cleaned and subsequently etched in 25% tetramethylammonium hydroxide. Secondly, the chemically treated wafers are then placed into a large plastic container filled with a mixture of HF and hydrochloric acid, and then centered over an inductive coil for photoconductance (PC) measurements. Thirdly, to inhibit surface recombination and measure the bulk lifetime, the wafers are illuminated at 0.2 suns for 1 min using a halogen lamp, the illumination is switched off, and a PC measurement is immediately taken. By this procedure, the characteristics of bulk silicon defects can be accurately determined. Furthermore, it is anticipated that a sensitive RT surface passivation technique will be imperative for examining bulk silicon defects when their concentration is low (<10¹² cm^-3).

Introduction

High lifetime (>1 msec) monocrystalline silicon is becoming ever more important for high efficiency solar cells. Understanding the recombination characteristics of embedded impurities has been, and remains an important topic. One of the most widely used techniques to examine the recombination activity of grown-in defects is by a photoconductance method¹. By this technique it is often difficult to completely separate surface from bulk recombination, thus making it difficult to examine the recombination characteristics of grown-in defects. Fortunately there exist several dielectric films which can achieve very low effective surface recombination velocities (S_eff) of < 5 cm/sec, and thus effectively inhibit surface recombination. These are, silicon nitride (SiN_x:H)², aluminum oxide (Al₂O₃)³ and amorphous silicon (a-Si:H)⁴. The deposition and annealing temperatures (~400 °C) of these dielectric films are considered to be low enough not to permanently deactivate the recombination activity of the grown-in defects. Examples of this are the iron-boron⁵ and boron oxygen⁶ defects. However, recently it was found that vacancy-oxygen and vacancy-phosphorus defects in n-type Czochralski (Cz) silicon can be completely deactivated at temperatures of 250-350 °C^7,8. Similarly a defect in float-zone (FZ) p-type silicon was found to deactivate at ~250 °C⁹. Therefore, conventional passivation techniques such as plasma enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD) may not be suitable for inhibiting surface recombination to examine grown-in bulk defects. Furthermore, SiN_x:H and a-Si:H films have been shown to deactivate bulk silicon defects through hydrogenation^10,11. Therefore to examine the recombination activity of grown-in defects, a RT surface passivation technique would be ideal. Wet chemical surface passivation fulfils this requirement.

In the 1990s Horanyi et al. demonstrated that immersion of silicon wafers in iodine-ethanol (I-E) solutions provides a means to passivate silicon wafers, achieving S_eff < 10 cm/sec¹². In 2007 Meier et al. showed that iodine-methanol (I-M) solutions can reduce the surface recombination to 7 cm/sec¹³, while in 2009 Chhabra et al. demonstrated that S_eff of 5 cm/sec can be attained by immersing silicon wafers in quinhydrone-methanol (Q-M) solutions^14,15. Despite the excellent surface passivation achieved by I-E, I-M and Q-M solutions, they do not provide adequate surface passivation (S_eff <5 cm/sec) to measure the bulk lifetime of high purity silicon wafers.

Another means to achieve a high level of surface passivation is by immersing silicon wafers in HF acid. The notion of using HF to passivate silicon wafers was first introduced by Yablonavitch et al. in 1986, who demonstrated a record low S_eff of 0.25±0.5 cm/sec¹⁶. Although excellent surface passivation was attained on high resistivity wafers, we have found the technique to be non-repeatable, thus adding a large uncertainty to the lifetime measurement. Therefore to limit the uncertainty by consistently achieving a very low S_eff (~1 cm/sec), we have developed a new HF passivation technique that incorporates three critical steps, (i) chemically cleaning and etching of silicon wafers, (ii) immersion in a 15% HF solution and (iii) illumination for 1 min^17,18. This procedure is both simple and time efficient in comparison to the traditional PECVD and ALD deposition techniques listed above.

Protocol

1. Experimental Setup

1. Locate a suitable fume hood for the measurement technique, and remove any irrelevant equipment to allow better air flow and reduce cluttering. Do not use any chemicals other than hydrofluoric acid (HF) in the fume hood.
2. Test the quality of the deionized (DI) water from the tap within the fume hood using a conductivity meter. Ensure that the DI water has a conductance of at most 0.055 µS/cm at a temperature of 20 °C.
3. Place a minority carrier lifetime tester into the fume hood. Connect the cables to a computer, which is situated on a table outside of the fume hood.
4. Switch on the computer and lifetime tester. Open the lifetime tester file on the computer and click the 'measure' button to ensure correct communication between computer and the lifetime tester. The light source on the lifetime tester should flash if the computer and tester have been connected correctly.
5. Place a halogen lamp inside the fume hood. Position the lamp such that it can illuminate the lifetime tester stage where the sample will be located.
6. Connect the halogen lamp to a power source which should be situated outside of the fume hood. When the halogen lamp is switched on, its intensity should be at least 0.02 W/cm² on the lifetime tester stage.

2. Preparing the 15% HF Solution

Note: HF is a dangerous chemical and must be treated with care. It causes slow, sustained, and deep damage to the body following exposure. HF does not readily burn the skin like other acids - rather it absorbs quickly into the skin and causes deep blistering and damage to bones. This means that bones become brittle and blistered as the fluorine reacts with calcium. HF also binds with free calcium which is used in nerve regulation and osmotic cell balance, so binding of free calcium in the body can be fatal. It is paramount that the user follows laboratory safety protocols when using HF, and ensures they know the location of the HF first aid kit and hexafluorine (or calcium gluconate gel).

1. Place a chemically cleaned round plastic container (H: 55 mm, D: 170 mm) into the fume hood (same as in Section 1). The container must have a clear plastic lid. See Section 6 on how to chemically clean the container prior to use.
2. Clear the area around the plastic container in the fume hood so the HF solution can be prepared without any obstructions nearby.
3. Apply all personal protection equipment (PPE).
4. CAUTION: Add 50 ml of HF to 100 ml DI water (H₂O) in the container.
5. CAUTION: Add 20 ml of hydrochloric acid (HCl) to the container and mix the H₂O : HF : HCl solution using plastic tweezers. Rinse the tweezers thoroughly afterwards.
6. CAUTION: Place the lid on the plastic container and allow the solution to settle for 1 hr. During this time, HF fumes will condense on the lid.
7. Adequately label the container, and make it clear that it contains HF.
   Note: The HF solution will last 1-2 months with heavy usage. Therefore there is no need to replace the solution each time a measurement is to be performed.

3. Calibration of the Lifetime Tester

1. Locate at least 6 silicon wafers of known resistivity and conductance. Ideally the resistivity range should span 0.1-100 Ω-cm.
2. On the computer, open the lifetime tester calibration file. Enter the resistivity of each wafer in the table and then click 'Update # of wafers'.
3. Click the 'Get data' button in the calibration file and enter the details of the lifetime tester.
4. Apply all necessary PPE.
5. CAUTION: Place the plastic container filled with the HF solution on the lifetime tester stage and position it over the inductive coil (blue circle region).
6. When prompted to measure the 'air voltage' on the computer, measure the voltage of the HF solution by clicking the 'Ok' button. In this case, the air has been replaced by the conductance of the HF solution.
7. CAUTION: Carefully remove the container filled with HF from the stage and place it on the fume hood bench. Carefully remove the lid.
8. CAUTION: When the lid has been removed from the container, immerse the first silicon wafer into the HF solution using plastic tweezers. Place the lid back on the container.
9. CAUTION: Place the plastic container back onto the lifetime tester stage and position it over the inductive coil. Ensure the silicon wafer is centered over the coil (blue circle region).
10. When prompted to measure the 'sample voltage' on the computer, click the 'Ok' button once again. Remove the container from the stage and place it on the fume hood bench.
11. CAUTION: Carefully remove the silicon wafer from the HF solution using plastic tweezers and rinse the wafer in the dedicated rinse beakers. Do a final rinse under the tap in the fume hood.
12. CAUTION: Place the lid back onto the container, and position the container back onto the lifetime tester stage.
13. Repeat steps 3.6 to 3.12 until all samples have been measured.
14. Once all of the samples have been measured, click the 'Fit data' button in the calibration file. This will fit a parabolic curve to the measured data and provide calibration coefficients A, B and C which are specific to this setup.
15. On the computer, open the lifetime tester file and click on the 'settings' tab. Enter in the new calibration coefficients A, B and C for the HF setup. Save the file under a new name.
    Note: The HF setup only requires a calibration every 6 months. Therefore there is no need to calibrate the setup each time a measurement is to be performed.

4. Wet Chemical Treatment of Silicon Wafers Prior to Measuring

1. Prepare standard clean 1 (SC 1).
   1. CAUTION: In an alkaline allocated fume hood, add 185 ml of ammonium hydroxide (NH₄OH) to 1,295 ml of DI water in a 2 L glass beaker.
   2. CAUTION: Place the beaker onto a hot plate and heat the H₂O : NH₄OH solution to a temperature of ~50 °C. Use a watch glass to cover the beaker.
   3. CAUTION: Once the H₂O : NH₄OH solution has reached a temperature of ~50 °C, add 185 ml of hydrogen peroxide (H₂O₂) and continue to heat until the temperature reaches ~75 °C. This H₂O : NH₄OH : H₂O₂ solution is known as SC 1.
      Note: SC 1 should be changed daily to effectively clean silicon wafers.
2. Prepare standard clean 2 (SC 2).
   1. CAUTION: In an acid allocated fume hood, add 185 ml of HCl to 1,295 ml of DI water in a 2 L glass beaker.
   2. CAUTION: Place the beaker onto a hot plate and heat the H₂O : HCl solution to a temperature of ~50 °C. Use a watch glass to cover the beaker.
   3. CAUTION: Once the H₂O : HCl solution has reached a temperature of ~50 °C, add 185 ml of H₂O₂ and continue to heat until the temperature reaches ~75 °C. This H₂O : HCl:H₂O₂ solution is known as SC 2.
      Note: Change SC 2 daily to effectively clean silicon wafers.
3. Prepare the silicon etching solution.
   1. In an alkaline allocated fume hood, add 1,600 ml of tetramethylammonium hydroxide (TMAH) to a chemically clean 2 L glass beaker. Please see Section 6 on how to chemically clean the beaker prior to use.
   2. Place the beaker onto a hot plate and heat the TMAH solution to a temperature of ~85 °C. Use a watch glass to cover the beaker.
      Note: The TMAH solution will not need changing until it starts to crystallize, i.e., after 1 month.
4. Perform the wet chemical treatment.
   1. Load samples into a quartz cradle and place them in a communal HF solution in the lab. The concentration is not critical.
   2. After ~10 sec or once the samples become hydrophobic (pull dry), remove the cradle from the HF solution and rinse using 3 beakers filled with DI water.
   3. Transport the cradle of wafers to the fume hood where SC 1 has been prepared. When SC 1 has stabilized at ~75 °C, slowly immerse the cradle of wafers into SC 1.
   4. Clean the wafers in SC 1 for 10 min.
   5. After 10 min has elapsed, remove the cradle of wafers from SC 1 and rinse them using three 2 L glass beakers filled with DI water. Chemically clean these glass beakers prior to filling them with DI water. Please see Section 6 on how to chemically clean the beakers.
   6. After rinsing, place the samples into a HF solution which is dedicated only for post SC 1 processing. The concentration is not critical.
   7. After ~10 sec or once the samples become hydrophobic (pull dry), remove the cradle from the HF solution and rinse using 3 beakers filled with DI water. Use the same beakers as in step 4.4.5.
   8. Transport the cradle of wafers to the fume hood where SC 2 has been prepared. When SC 2 has stabilized at ~75 °C, slowly immerse the cradle of wafers into SC 2.
   9. Clean the wafers in SC 2 for 10 min.
   10. After 10 min has elapsed, remove the cradle of wafers from SC 2 and rinse them using 3 beakers filled with DI water. Use the same beakers as in step 4.4.5.
       Note: The cradle of wafers could be stored in one of the rinse beakers filled with DI water until the following day.
   11. After rinsing, place the samples into a HF solution which is dedicated only for post SC 2 processing. The concentration is not critical.
   12. After ~10 sec or once the samples become hydrophobic (pull dry), remove the cradle from the HF solution and rinse using 3 beakers filled with DI water. Use the same beakers as in step 4.4.5.
   13. Transport the cradle of wafers to the fume hood where the TMAH solution has been prepared. When the TMAH solution has stabilized at ~85 °C, slowly immerse the cradle of wafers into the TMAH solution.
   14. Etch the wafers in TMAH for 5 min. This will remove ~5 µm of silicon.
   15. After 5 min has elapsed, remove the cradle of wafers from the TMAH solution and rinse them using 3 beakers filled with DI water. Use the same beakers as in step 4.4.5. More than 3 rinses might be required as TMAH is quite 'sticky'.
   16. Transport the cradle of wafers in DI water to the fume hood where the experiment has been setup. See section 1.1.
   17. Measure the prepared silicon wafers within 2 hr after step 4.4.16.

5. Measurement Procedure

1. Apply all necessary PPE.
2. Fill two 2 L plastic beakers with DI water and place them in the fume hood. These will be used for rinsing the silicon samples post measurement.
3. Place plastic tweezers into an empty 500 ml plastic beaker and place inside the fume hood near the rinse beakers. These tweezers will be used to handle the silicon samples.
4. On the computer, open the lifetime tester file. Make sure the file contains the correct calibration coefficients for the HF measurement setup. See section 3.
5. In the lifetime tester file, select the 'Transient' mode. Enter the details of the silicon wafer to be measured, for example, thickness, resistivity and dopant type.
6. CAUTION: Place the container (lid on) filled with HF onto the stage of the lifetime tester and center it over the inductive coil (blue circle region). Let the solution settle for 1 min.
7. On the computer, click on the 'Zero instrument' button in the lifetime tester file. This will measure the voltage of the solution.
   Note: Over time, the solution voltage will decrease because the composition of the solution changes with time. Irrespective of this, the HF solution does not show any degradation in its passivating quality, which is why the solution can be used for 1-2 months without being changed.
8. CAUTION: Carefully remove the container from the lifetime tester stage and place it on the fume hood bench.
9. CAUTION: Carefully remove the lid from the container filled with the HF solution. If there is some condensation on the lid, carefully rinse the lid in DI water using the tap in the fume hood.
10. CAUTION: Carefully immerse the first silicon wafer into the HF solution. Using the plastic tweezers, lightly press down on the silicon wafer to ensure it is sitting on the bottom of the container.
11. Rinse the tweezers and place them back into the empty 500 ml plastic beaker.
12. CAUTION: Carefully place the lid back on the container and then place the container on the lifetime tester stage. Ensure the silicon wafer is positioned over the inductive coil (blue circle region).
13. Rinse and dry your gloves. Remove them before operating the computer.
14. If the fluorescent light in the fume hood is on, it must be switched off before a measurement is to be conducted. Minimum light is required during the measurement.
15. Switch the halogen lamp on, and ensure it is illuminating the silicon wafer through the lid of the plastic container. Time for 1 min (the exact time is not critical).
16. During the illumination period, click the 'Measure' button in the lifetime file on the computer. A small window labeled 'Taking data' will appear.
17. In the 'Taking data' window, type in a name for the file. Under 'sample averaging' select 10.
18. When the silicon wafer has been illuminated for 1 min, switch the halogen lamp off and immediately click the 'Average' button in the 'Taking data' window. The lifetime tester will flash 10 times and average the lifetime measurements.
    Note: Once the halogen lamp is switched off, the passivation of the silicon wafer will start to degrade, and thus it is important to measure immediately after illumination to achieve the best results.
19. When the averaging is complete, click the 'Ok' button in the 'Taking data' window.
20. If another measurement is required, repeat steps 5.15-5.18.
21. CAUTION: Once the measurement is complete, remove the container from the lifetime tester stage and place it on the fume hood bench.
22. CAUTION: Carefully remove the lid from the container filled with the HF solution. If there is some condensation on the lid, carefully rinse the lid in DI water using the tap in the fume hood.
23. CAUTION: Carefully remove the silicon wafer from the HF solution using plastic tweezers and rinse the wafer in the dedicated rinse beakers. Do a final rinse under the tap in the fume hood.
24. If more samples are to be measured, repeat steps 5.10-5.23.
25. Once all samples have been measured, ensure the lid is placed on the container and store the HF solution in the fume hood. Make sure the container is adequately labeled.
26. Rinse all beakers and tweezers and store them in their allocated spots in the laboratory.
27. Allow the lifetime tester to remain in the fume hood for 1 hr after use and then remove it from the fume hood.

6. Chemical Cleaning of Beakers and Containers

1. In an alkaline allocated fume hood, prepare an SC 1 solution as outlined in sections 4.1.1-4.1.4.
2. To chemically clean beakers and containers, pour the SC 1 solution into the beakers/containers. The beakers can only be cleaned one at a time.
3. Let the solution clean the beaker/container for 10 min. If the beaker/container is plastic, it cannot be placed on a hot plate, and therefore the SC 1 solution will cool during the clean. This will not affect the cleaning process.
4. After 10 min, pour the SC 1 solution into another beaker/container if they require cleaning also. Otherwise let the SC 1 solution cool in a large beaker. Once cooled, SC 1 can be poured down the sink with running water.
5. Rinse SC 1 cleaned beaker/container in DI water.
6. In an acid allocated fume hood, prepare an SC 2 solution as outlined in sections 4.2.1-4.2.4.
7. Pour the SC 2 solution into beaker/container from step 6.5. Allow the solution to clean the beaker/container for 10 min.
8. After 10 min, pour the SC 2 solution into another beaker/container if they require cleaning also. Otherwise let the SC 2 solution cool in a large beaker. Once cooled, SC 2 can be poured down the sink with running water.
9. Rinse SC 2 cleaned beaker/container in DI water. The beaker/container is now chemically cleaned.

Results

Figure 1a shows a schematic and Figure 1b shows a photograph of the experimental setup. When a silicon wafer is immersed into the HF solution, subsequently placed onto the lifetime tester stage and a measurement is performed (before illumination), a lifetime curve which is limited by surface recombination will result, as shown by the blue triangles in Figure 2. However, when the sample is illuminated for 1 min (while immersed in HF), as shown in Figure 1...

Discussion

The successful implementation of the bulk silicon lifetime measurement technique described above is based on three critical steps, (i) chemically cleaning and etching the silicon wafers, (ii) immersion in a 15% HF solution and (iii) illumination for 1 min^17,18,19. Without these steps, the bulk lifetime cannot be measured with any certainty.

As the measurement technique is conducted at RT, the surface passivation quality is highly susceptible to surface contamination (metals, organic...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Hydrofluoric acid (48%)	Merck Millipore,   http://www.merckmillipore.com/AU/en/product/Hydrofluoric-acid-48%25,MDA_CHEM-100334	1003340500	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
Hydrochloric acid 32%, AR	ACI Labscan, http://www.rcilabscan.com/modules/productview.php?product_id=1985	107209	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
Ammonia (30%) Solution AR	Chem-supply, https://www.chemsupply.com.au/aa005-500m	AA005	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
Hydrogen Peroxide (30%)	Merck Millipore, http://www.merckmillipore.com/AU/en/product/Hydrogen-peroxide-30%25,MDA_CHEM-107209	1072092500	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
Tetramethylammonium hydroxide (25% in H2O)	J.T Baker, https://us.vwr.com/store/catalog/product.jsp?product_id=4562992	5879-03	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
640 ml round plastic container	Sistema, http://sistemaplastics.com/products/klip-it-round/640ml-round		This is a good container for storing the 15% HF solution in.
WCT-120 lifetime tester	Sinton Instruments, http://www.sintoninstruments.com/Sinton-Instruments-WCT-120.html
Dell workstation with Microsoft Office Pro, Data acquisition card and software including Sinton Software under existing license.	Sinton Instruments, http://www.sintoninstruments.com
Halogen optical lamp, ELH 300 W, 120 V	OSRAM Sylvania, http://www.sylvania.com/en-us/products/halogen/Pages/default.aspx	54776	Any equivalent lamp could be used.
Voltage power source	Home made power supply		Any power supply could be used provided it can produce up to 90 Volts and 1-5 Amps.
Conductivity meter	WTW, http://www.wtw.de/uploads/media/US_L_07_Cond_038_049_I_02.pdf	LF330