PVfactory 3 – Acidic Texturing

Introduction

Texturing is used to reduce the reflection of light from the front surface of wafers. Multicrystalline wafers are routinely textured using acidic solutions containing hydrofluoric acid (HF) and nitric acid (HNO3) in an inline process. These solutions etch all silicon crystal planes at approximately the same rate and so consequently are called isotropic etchants. A textured surface results because etching occurs preferentially at defects on the surface remaining after wafer sawing. The resulting surface appears as randomly-arranged concavities. The texturing performance depends on the HF and HNO3 concentrations, belt speed and bath temperature.

Learning Objectives

  • Explain why you should not use alkaline texturing to texture multi-crystalline wafers
  • Understand how acidic texturing works and the roles of the main chemicals required
  • Be able to perform a main factor experiment to determine the most important parameter to optimise
  • Be able to perform a single factor experiment to optimise the acidic texturing process
  • Understand how the wafer sawing method affects acidic texturing

Tutorial Exercise

Like Alkaline Texturing, Acidic Texturing performance depends on many parameters, including HF and HNO3 concentration, bath temperature and belt speed. Note that etching time has been replaced by belt speed. The etching bath is a fixed length and so the belt speed is used to tune the etching time. Please review the PVmanufacturing.org page on Acidic Texturing and the help files in PV Factory before attempting this tutorial so that you understand the effect of the different parameters. For this tutorial exercise you will use the Factory Default settings for all processes except for the Acidic Texturing step. All experiments should use batches of wafers (at least 10 per batch) with the following properties:

  1. Standard multicrystalline silicon wafers;
  2. 200 mm thick;
  3. Resistivity of 1 W cm; and
  4. Cut using a standard wire saw (this assumes a wire saw that uses a slurry not diamond tips).

You can save these batch settings to your recipe (use the “S” icon on the left hand screen). PV Factory does not let you do Saw Damage Removal and Alkaline Texturing on multi-crystalline wafers and so when you select multi-crystalline wafers you will be taken directly to the Acidic Texturing step. Once you have set the parameters for the Acidic Texturing, run that step and before you press Complete Batch to run the remaining steps, record the etched silicon thickness per side and download the Reflection data from the Characterisation Lab.

Instead of measuring a single reflectance value at 600 nm, this week you will need to calculate a Weighted Average Reflectance, WAR, for your experiments. This takes into account the reflectance of all wavelengths in the 300 to 1200 nm range of interest, R(λ), and weights it according to the spectral irradiance, I(λ) and the internal quantum efficiency, Q(λ) :

WAR=\frac{\int I(\lambda)Q(\lambda)R(\lambda)d \lambda}{\int I(\lambda)Q(\lambda)d\lambda}

Data for I(λ) and Q(λ) are in the provided spreadsheet, PVfactory Tutorial 3 – Reflectance Worksheet. The Q(λ) values are for a typical multi-crystalline cell fabricated in PV Factory. You can download R(λ) from PV Factory and paste this data into the provided spreadsheet to calculate the WAR. You will also record the short circuit current density, JSC, as you did last week, to determine the effect of the texturing on cell performance.

So the procedure you will need to follow this week is:

Create New Batch (standard multi-crystalline wafers) → Acidic Texture (this optimisation) → Return to Acidic Texturing step and record how much silicon was etched per side → Go to Characterisation Lab and download the reflectance data for a wafer (non-destructive measurement) → Complete Batch and record the batch average JSC.

Part 1 – Main Factor-Response Experiment

For this tutorial you will need to design your own main factor response experiment. You can use your spreadsheet from the last tutorial as a template.

Pre-Work

Design a main factor response experiment which explores the effects of the following factors of interest (allowed ranges in PV Factory are in parentheses):

  • HF concentration (5 – 30 % (v/v));
  • HNO3 concentration (5 – 60 % (v/v));
  • Bath temperature (5 – 25 °C); and
  • Belt speed (0.5 – 3 m min-1)

on the responses of amount of silicon etched per side (mm), WAR (%), and JSC (mA/cm2). The suggested factor settings are listed in the table below, but you can also select other settings for your main factor response experiment.

Table 1 - Factor settings for main factor response experiment
Factor SettingsMain Factors for Acidic Texturing Process
[HF]
% (v/v)		[HNO3]
% (v/v)		Temperature
(oC)		Belt Speed
(m.min-1]
-104050.5
02050101.5
+3060152.0

As described in last week’s tutorial, “-“, “0” and “+” are used to indicate “a lower setting”, “the baseline setting” (or “default”) and “a higher setting” for the factors, respectively. The actual values for these recipe settings are provided in the table above.

Main Factor Response Experiment: Activities and Questions

  1. If you are completing this exercise as a class, get your tutor to check your experimental design before you start your simulations
  2. Before you start the experiments, process 10 batches to completion to stabilise the bath. Why do you do this?
  3. Then process batches for each of your designed experiments.
  4. Produce a main factor response graph for each of the responses.
  5. Which of the factor(s) is/are most significant in determining the WAR and JSC?
  6. Did you really need to fabricate complete cells to determine the main factor(s)?
  7. Indicate the baseline point in the ternary iso-etch graph below.
  8. Are your trends in HF and HNO3 concentration consistent with what is predicted by the iso-etch graph (taking into account a bath length of 2.2 m)?
Figure 1 – Iso-etch graph to determine the etch rate from HF:HNO3:H2O ratio

Part 2 – Single Factor Response Curve

Identify the factor that you think has the greatest effect on the WAR and run a single factor experiment to determine your preferred setting for that factor. We’ll also record the other responses that you considered in Part 1.

Single Factor Response Curve Experiment: Activities and Questions

  1. Use the same wafer parameters that you used for Part 1.
  2. Create and process at least 8 experimental batches (with at least 10 wafers per batch), varying the factor of interest over the range of values allowed by the PV Factory.
  3. Record all 3 responses for each experiment.
  4. Sketch an X-Y scatter plot for each response (y-axis) versus your factor of interest (x-axis).
  5. If the range of parameters allowed by PV Factory indicates the range of allowable settings in your inline tool would you suggest that the equipment provider makes any changes to the tool?
  6. Store your best recipe in PV Factory so you can use it for the following tutorials.

Part 3 – Understanding Acidic Texturing

The following questions are used to determine your understanding of the acidic texturing process. You may prepare your answers while completing the above tasks.

General questions:

  1. Why isn’t alkaline texturing used for multi-crystalline wafers?
  2. What is the role of nitric acid in acidic texturing?
  3. What is the role of hydrofluoric acid in acidic texturing?
  4. Why is hydrofluoric acid such a hazardous chemical?
  5. Why can small holes, such as shown in the scanning electron microscope image below, form in acidic textured surfaces?
Figure 2 – SEM image of an acidic textured surface of mc-Si, showing holes.

True/False questions:

  1. Wafers are immersed in KOH after etching to roughen the surface (by alkaline texturing).
  2. Acidic etching in solutions containing hydrofluoric and nitric acid etch (100) crystal planes more than 10 times faster than (111) crystal planes.
  3. Acidic texturing results in a lower minimum reflectance on multi-crystalline silicon wafers than alkaline texturing achieves on mono-crystalline silicon wafers.
  4. The WAR will be higher if you remove the saw damage before doing acidic texturing compared to the process followed in this tutorial.