PVfactory 1 – Saw Damage Removal Etch

Introduction

Wire sawing creates damage in the near surface region which needs to be removed in the solar cell manufacturing process. Failure to do so will result in reduced minority carrier lifetimes and consequently lower solar cell efficiencies. A commonly-used process to remove the damaged regions from each wafer surface is to use alkaline etching. The rate of etching depends on the bath temperature, etching time, and sodium hydroxide (NaOH) concentration.

Learning Objectives

• Eplain why wafers must be etched to remove saw damage
• Understand the basics of alkaline etching
• Perform a main factor experiment to determine the most important parameter to optimise
• Perform a single factor experiment to optimise the saw damage etch process

Tutorial Exercise

To determine which parameter(s) is most important to optimise, a main factor response experiment should first be performed. For the experiments carried out in this tutorial, you will need to use PV Factory. Make sure to read the page, Saw Damage Etching, before attempting the exercises to have a better understanding of the factors.

Because we are starting our optimisation process, you should go into the ‘Office’ and select ‘User Settings’. Make sure that you are set up to ‘Use Factory Default’. As we progress through this course, you will develop your own standard recipe, i.e. ‘Use my recipe‘.

To create new batches you need to select the ‘Office’ tab and then select ‘Start New Batch’. Choose to process at least 20 wafers which have the following properties:

1. Standard Cz mono-crystalline silicon wafers
2. 200 µm thick
3. Resistivity of 1 Ω cm
4. Cut using a standard wire saw (this assumes a wire saw that uses a slurry not diamond tips).

Once you have selected parameters for your saw damage removal etch, run the process by selecting the ‘Run Production Step’. Immediately after the process is completed, you can check the etched thickness of your wafers. To determine the effect of your Saw Damage Removal Etch on your cell efficiency, select to ‘Run Remaining Steps’. This will result in your batch to be processed using the default parameters. The I-V results for your batch will then be available.

Part 1 – Main Factor Response Experiment

A Main Factor Response experiment explores several factors of interest
(e.g., bath temperature, etching time and NaOH concentration) for one or more responses (e.g., average silicon removed per side, yield and mean cell efficiency). We can make a graphical model of the main factor response using a simple diagram that describes how a response behaves with changes in a factor. For example, the plot below shows a main factor response plot for three factors A, B and C (refer to Week 2 Tutorial).

In this plot, the “-“, “0” and “+” for the factor settings indicate “a lower setting”, “the baseline setting” (or “default”) and “a higher setting” for the factors. The actual values for these recipe settings are determined by the person carrying out the experiment. The suggested values for the factor settings for Part 1 are shown in the table below

Conduct a Main Factor Response Experiment

1. Before you start this experiment, process 3 batches of 20 cells to completion. Why?
2. Then process batches for each of the following experiments and complete the Table 1. Once you run the ‘Saw Damage Etch’ and record the etched wafer thickness, you can press the ‘Complete Batch’ button to complete processing the cells in the batch according to the Factory default settings.
3. For each batch record the following:
   1. Average silicon thickness removed per side (in µm)
   2. Yield (%)
   3. (Batch) Mean Cell Efficiency (%)
4. Produce a Main Factor Response graph for: (i) average change in wafer thickness; (ii) yield (%); and (iii) mean cell efficiency.

Questions:

1. What settings are required to achieve > 99% mechanical yield?
2. Which of the factors that you tested appear not to have a significant effect?
3. Which of these factors has a significant effect on the average change in wafer thickness?

Part 2 – Single Factor Response Curve

A Single Factor Response curve is the dependent-independent scatter plot that you are familiar with from engineering and science laboratories. One response is plotted against one factor on an X-Y scatter plot format. These plots are useful when we expect a cause-effect relationship between our factor and response, or a correlation between responses. In this exercise you will try to discover the cause-effect relationship between one factor and the responses of: (i) average silicon removed per side; (ii) yield %; and (iii) mean cell efficiency. Choose one factor to investigate from the ‘Saw Damage Removal Etch’ process (i.e., etch time, NaOH concentration, etch temperature) based on your analysis in Part 1.

Conduct a Single Factor Response Curve Experiment

1. Use the same wafer parameters as you used for Part 1. You can decide on how many wafers to process per batch but more wafers will take longer to process.
2. Create and process at least eight experimental batches (of at least ten wafers per batch), varying the factor of interest over the range of values allowed by the PV Factory. For example, you might create eight batches with the temperature varying from 20 to 90 ºC, stepping by 10 ºC.
3. Record the average change in wafer thickness, yield % and mean and standard deviation of cell efficiency for all batches. Note: you will need to download the I-V data and use Microsoft Excel to compute the standard deviation of the cell efficiency.
4. Construct an X-Y scatter plot of the Average Silicon Thickness Removed per Side (y-axis) as a function of your factor of interest (x-axis).
5. Construct an X-Y scatter plot of the Mean Cell Efficiency (y-axis) as a function of your factor of interest (x-axis). Show the 95 % confidence interval for the data points as error bars
6. On the graph constructed in (5) above add the Yield % using a secondary axis.
7. Save your best recipe in PV Factory so you can use it for the following tutorials.

Questions

1. How much silicon do you need to etch from each surface so as to not impact cell efficiency?
2. When do you know that you are removing too much silicon and how do you optimise this process?
3. Explain the variability that you observe in your results?
4. What was the highest mean cell efficiency that you recorded?
5. What is the purpose of the metric “Mean Efficiency X Yield” and is it useful?

Part 3 – Understanding Saw Damage Removal Etch

General questions:

1. What is the advantage of increasing temperature for increased silicon etching?
2. What is the advantage of increasing etching time for increased silicon etching?
3. Draw a curve of etch rate as a function of NaOH concentration. Why does it reduce at low and high concentration?
4. After simulating some batches with the same parameters does the “average silicon thickness removed per side” change with the number of batches processed, and if so why?

True/ False Questions:

1. Etching of monocrystalline silicon in NaOH solutions is anisotropic. TRUE / FALSE
2. Increasing etching time increases yields. TRUE / FALSE
3. It is not necessary to remove as much silicon from each surface if you are using a diamond-tipped wire saw to cut your wafers from the ingot (compared to using a wire saw that uses a slurry). TRUE / FALSE
4. Silicate build-up in the bath reduces the etch rate and can result in insufficient silicon being etched from each surface. TRUE / FALSE
5. For optimal texturing, the wafer surface before texturing must be polished. TRUE / FALSE