Semiconductor Fabrication: From Sand to Chip · Module 3
The Cleanroom and Contamination Control
A single speck of dust can ruin a chip. This module explains how fabs keep particles, metals, and other contaminants away from the wafer.
What you will learn
- Why contamination matters so much at the nanometer scale
- How a cleanroom is built and how its cleanliness is classified
- How wafers are cleaned between process steps
- How defects determine the yield of working chips
In plain language
The features on a chip are thousands of times smaller than the width of a human hair. At that scale, a dust particle too small to see is the size of a boulder. If one lands on the wafer at the wrong moment, it can block part of a pattern or join two wires that should be separate, and the chip fails.
A fab therefore filters its air without pause, dresses its people in suits that cover them from head to foot, and washes each wafer many times in very pure water and chemicals. The air in a chip fab holds far fewer particles than the air in a hospital operating room.
ANNLab, University of Arizona
Figure 1. Sizes of common particles compared with the smallest features on a leading chip. The axis is logarithmic.
Core lesson
Kinds of contamination
Particles are the most visible problem. They come from people, from moving parts in equipment, and from the process chemicals themselves. A person sitting still sheds on the order of 100,000 small particles every minute, and many more when walking.
Metal atoms are a second problem. Sodium ions can move through the gate insulator and shift the threshold voltage of a transistor. Iron and copper in silicon shorten the lifetime of carriers and increase leakage. Organic films such as oils and resist residue stop later layers from sticking. A thin oxide that forms in air, called native oxide, can spoil a contact between two layers.
How a cleanroom works
Air enters through filters in the ceiling. A HEPA filter removes 99.97% of particles 0.3 µm in size, and ULPA filters do better still. The air moves straight down in a smooth, even stream called laminar flow, leaves through the floor, and is filtered again. The room is kept at a slightly higher pressure than the space around it, so any leak pushes clean air out and never draws dirty air in. Temperature and humidity are held within narrow limits.
People wear a coverall, hood, boots, gloves, and a face mask, often called a bunny suit. The suit protects the wafers from the person, not the person from the wafers. Lithography areas use yellow light because photoresist reacts to blue and ultraviolet light.
In modern 300 mm fabs, wafers travel in sealed carriers called FOUPs that hold 25 wafers and dock directly to each tool. An overhead transport system moves the carriers, so the wafers rarely meet the room air at all.
Cleanroom classes
Cleanliness is rated by the ISO 14644 standard. The class number sets the largest number of particles allowed in a cubic meter of air. A lower class number means cleaner air. The NanoFabrication Core Facility at the University of Arizona is an ISO 5 cleanroom.
| ISO class | Particles per cubic meter | Older name |
|---|---|---|
| ISO 3 | 35 | Class 1 |
| ISO 4 | 352 | Class 10 |
| ISO 5 | 3,520 | Class 100 |
| ISO 6 | 35,200 | Class 1,000 |
| ISO 7 | 352,000 | Class 10,000 |
| ISO 9 (ordinary room air) | 35,200,000 | not classified |
Cleaning the wafer
Wafers are cleaned before every high-temperature step and after many others. The standard sequence, developed at RCA in the 1960s, has two main baths. The first is a warm mixture of ammonium hydroxide, hydrogen peroxide, and water, which removes organic films and particles. The second is a mixture of hydrochloric acid, hydrogen peroxide, and water, which removes metals. A short dip in dilute hydrofluoric acid removes thin oxide when a bare silicon surface is needed.
Each bath is followed by a rinse in deionized water, which has had nearly all of its dissolved ions removed. These chemicals are hazardous, and hydrofluoric acid in particular requires special training and protective equipment.
Defects and yield
Yield is the fraction of chips on a wafer that work. One fatal defect anywhere on a chip ruins the whole chip. A larger chip offers a larger target, so its yield is lower at the same level of cleanliness. Fabs inspect wafers after key steps, count the defects, and trace each kind back to the tool or material that caused it. Raising yield by a few percent can decide whether a product makes money.
Going deeper
These sections add the equations behind the core lesson, each with a worked example. They assume first-year algebra and can be skipped without losing the thread of the course.
If defects fall at random with an average density D0 per unit area, the chance that a chip of area A has none is given by the Poisson model.
Y = e−A D0
Worked example
With D0 = 0.1 defects per cm2, a chip of 1 cm2 has a yield of e−0.1 = 90%. A chip of 4 cm2 made in the same fab has a yield of e−0.4 = 67%. Real fabs use more detailed models, because defects tend to cluster, but the trend is the same.
The ISO standard gives the limit for any particle size D, in micrometers, from the class number N.
C = 10N × (0.1 / D)2.08 particles per cubic meter
Worked example
For ISO 5 and particles of 0.5 µm, C = 105 × (0.2)2.08 = 105 × 0.0352 = 3,520 particles per cubic meter, the value in the table. The older name, Class 100, counted the same air as 100 such particles per cubic foot.
A common rule of thumb is that a particle larger than about half the smallest feature can cause a failure. For a pattern with 20 nm lines, particles of 10 nm matter. Particles this small are far below the wavelength of visible light, so finding them requires special inspection tools. This is one reason why cleanliness requirements tighten with each new technology generation.
Check your understanding
Try each question before opening the answer. The last question uses the Going deeper material.
It protects the wafers. People shed skin flakes, hair, and droplets, and the suit keeps these away from the wafers and the tools.
Air flows from high pressure to low pressure. With higher pressure inside, any gap or open door lets clean air flow out and stops unfiltered air from flowing in.
The first bath, with ammonium hydroxide and hydrogen peroxide, removes organic films and particles. The second, with hydrochloric acid and hydrogen peroxide, removes metals.
Defects fall across the wafer at roughly the same density everywhere. A larger chip covers more area, so it is more likely to contain at least one fatal defect.
Y = e−2 × 0.2 = e−0.4 = 67%.
Key terms
- Cleanroom. A room in which the number of airborne particles is controlled to a stated limit
- HEPA filter. A high-efficiency air filter that removes 99.97% of particles 0.3 µm in size
- Laminar flow. Air moving in smooth, parallel streams that carry particles away without mixing
- ISO class. A rating of cleanroom air based on the allowed number of particles per cubic meter
- FOUP. Front opening unified pod, a sealed carrier that holds 25 wafers
- RCA clean. The standard two-bath wet cleaning sequence for silicon wafers
- Deionized water. Water purified until almost no dissolved ions remain
- Native oxide. The very thin oxide that forms on silicon in air at room temperature
- Defect density. The average number of fatal defects per unit area of wafer
- Yield. The fraction of chips on a wafer that work
In the fab
Cleanroom technicians, facilities engineers, and yield engineers keep a fab running. Yield engineers act as detectives, linking a pattern of failed chips to one tool, one chemical, or one step.
Further reading
J. D. Plummer and P. B. Griffin, Integrated Circuit Fabrication: Science and Technology, Cambridge University Press. The authors offer a prepublication version that is free to read on the Stanford book site (opens in a new tab).
For a look inside a working cleanroom, see the Cleanroom Virtual Reality Tour and the Cleanroom Interactive Map on our Training page.
Previous: Module 2Course homeNext: Module 4, Lithography
Created by Prof. Zafer Mutlu, ANNLab, University of Arizona.