PE1-1.2 Silicon Wafer Preparation & Cleaning
You can compare Czochralski and float zone crystal growth, describe silicon shaping through its mechanical and chemical operations, and explain wet and dry wafer cleaning.
Everything downstream depends on starting with a near-perfect single crystal, and CZ versus FZ is a purity-against-size trade: FZ gives cleaner material, CZ gives the large wafers industry wants. Electronic grade silicon is refined to a purity that is hard to picture — impurities measured in parts per billion. Cleaning gets its own unit because at these dimensions a residue is a defect, and the RCA-style wet chemistry is one of the oldest unchanged recipes in the industry.
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Electronic grade silicon
The purity the process demands, which is measured in parts per billion. The number is worth pausing on because nothing else in engineering is made this pure.
Czochralski (CZ) method of single crystal growth
Pulling a single crystal from a melt, which is how nearly all wafers are made. The rotation and pull rate control the diameter, which is why wafers come in standard sizes.
Float zone (FZ) method of single crystal growth
The alternative, which avoids the crucible and so reaches higher purity. It is used where the device needs it and costs more, which is the trade to remember.
Silicon shaping: mechanical operations
Turning a cylindrical crystal into flat discs: sawing, lapping and polishing. Mechanical damage from these steps has to be removed before anything else happens.
Silicon shaping: chemical operations
The chemical half of the same job, including the etch that removes the damaged surface. It leaves the mirror finish that lithography later requires.
Basic concepts of wafer cleaning
Why cleaning gets a whole section: contamination at any step ruins everything after it. Wafers are cleaned before nearly every process step, not once at the start.
Wet-cleaning technology
The liquid chemistry, which is the traditional approach and still the workhorse. The standard sequences are named after the labs that devised them.
Dry-cleaning technology
The gas phase and plasma alternatives, which use less chemical and reach features the liquid cannot. The comparison with the item above is the point.
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