Semiconductor Fabrication: From Sand to Chip · Module 8
Thin-Film Deposition
Chips are built by adding thin layers as well as by removing them. This module covers the main ways to deposit a film.
What you will learn
- How physical and chemical deposition methods differ
- What epitaxy is and where it is used
- How atomic layer deposition builds a film one layer at a time
- How step coverage and temperature decide which method is used
In plain language
Many layers in a chip are not grown from the wafer. They are added on top. These include the metals for wires, the insulators between them, and the films that form the gate.
There are two families of methods. Physical methods carry atoms from a solid source to the wafer, either by boiling a metal in vacuum or by knocking atoms off a target with ions. This resembles spray painting. Chemical methods flow gases over a hot wafer, where they react and leave a solid film behind, somewhat as frost forms on a cold window.
One chemical method, atomic layer deposition, builds the film a single atomic layer at a time. It can coat the inside of a deep, narrow hole as evenly as a flat surface.
ANNLab, University of Arizona
Figure 1. A conformal film covers a trench evenly. A non-conformal film is thick on top and thin on the sidewalls and the bottom.
Core lesson
What makes a good film
Engineers judge a film by its thickness and uniformity, its composition, its internal stress, and how well it sticks. Two further properties decide which method can be used. The first is step coverage, the ratio of the film thickness on a sidewall to the thickness on the top surface. The second is the process temperature. Once metal wiring is on the wafer, later steps must stay at or below about 400 °C.
Physical vapor deposition
In evaporation, a metal is heated in a high vacuum, often by an electron beam, until it boils. The atoms travel in straight lines to the wafer. Surfaces that face the source are coated and sidewalls are not, so step coverage is poor. Research labs put this to use in a patterning method called lift-off, in which metal deposited on top of resist is washed away with the resist.
In sputtering, ions from an argon plasma strike a target made of the material to be deposited. Atoms knocked out of the target land on the wafer. Sputtering works for alloys and for metals with very high melting points, and it covers steps better than evaporation. Production fabs use it for metal layers and for the thin liner and seed layers in copper wiring.
Chemical vapor deposition
In chemical vapor deposition, or CVD, gases react on the hot wafer surface. Low-pressure CVD is done in a furnace between about 550 and 800 °C. It deposits polysilicon from silane, silicon nitride from dichlorosilane and ammonia, and silicon dioxide from a liquid source called TEOS. The films are uniform and cover steps well.
Plasma-enhanced CVD uses a plasma to supply the energy for the reaction, so films can be deposited at 200 to 400 °C. It is the standard method for insulators deposited after the metal wiring has started. CVD of tungsten from tungsten hexafluoride fills the narrow holes that connect the transistors to the first wiring layer.
Epitaxy
Epitaxy is the growth of a crystal film that continues the crystal structure of the wafer beneath it. Silicon epitaxy provides very pure starting layers. Films of silicon mixed with germanium are grown in the source and drain regions to strain the channel and raise its performance. Compound semiconductors for LEDs and lasers are grown by a form of CVD that uses metal-organic gases, and research labs often use molecular beam epitaxy in ultrahigh vacuum.
Atomic layer deposition
Atomic layer deposition, or ALD, splits a CVD reaction into two halves. The wafer is first exposed to one gas, whose molecules attach to every available site on the surface and then stop, because they do not react with each other. The chamber is purged. A second gas then reacts with the attached layer and prepares the surface for the next cycle. The chamber is purged again.
Each cycle adds the same small thickness, about 0.1 nm, on every exposed surface regardless of its shape. The film thickness is simply the number of cycles times the growth per cycle. Aluminum oxide grown from trimethylaluminum and water is the classic example. ALD is slow, so it is used for thin films where control matters most, such as high-k gate insulators.
| Method | Typical wafer temperature | Step coverage | Common uses |
|---|---|---|---|
| Evaporation | Near room temperature | Poor | Metals patterned by lift-off |
| Sputtering | Room temperature to 300 °C | Moderate | Metals, liners, seed layers |
| Low-pressure CVD | 550 to 800 °C | Good | Polysilicon, nitride, oxide |
| Plasma-enhanced CVD | 200 to 400 °C | Moderate to good | Insulators over metal |
| ALD | 100 to 350 °C | Excellent | Gate insulators, thin liners |
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.
Worked example
A 2 nm hafnium dioxide film is needed, and the process adds 0.1 nm per cycle. The recipe needs 20 cycles. If one cycle takes 4 seconds, the deposition takes 80 seconds. A 100 nm film by the same process would need 1,000 cycles and more than an hour, which is why ALD is kept for thin films.
At the temperatures used for low-pressure CVD, the deposition rate is set by the surface reaction and follows the Arrhenius law. It rises steeply with temperature.
rate ∝ e−Ea / kT
Worked example
Polysilicon deposition from silane has an activation energy of about 1.7 eV. Raising the temperature from 620 °C to 640 °C, which is from 893 K to 913 K, multiplies the rate by e0.48 = 1.6. A change of 20 degrees gives 60% more film. To hold the thickness within 1%, the furnace must hold its temperature within a fraction of a degree.
The mean free path of a gas atom is roughly 5 cm divided by the pressure in millitorr.
Worked example
An evaporator runs at 10−6 torr, which is 10−3 millitorr. The mean free path is about 5,000 cm, or 50 meters, far longer than the chamber. Evaporated atoms reach the wafer without a single collision, so any part of the surface that cannot see the source receives no film.
Check your understanding
Try each question before opening the answer. The last question uses the Going deeper material.
In physical vapor deposition, atoms of the film material travel from a solid source to the wafer. In chemical vapor deposition, gases react on the wafer surface and the film is a product of that reaction.
Low-pressure CVD needs 550 °C or more, which the wiring cannot survive. The plasma supplies the energy for the reaction, so the film can be deposited at 400 °C or below.
Each half of the cycle stops by itself once the surface is covered, so every cycle adds the same thickness. The thickness is set by counting cycles.
ALD. Its surface reactions reach every exposed surface and stop after one layer, so the sidewalls and the bottom receive the same thickness as the top.
5 / 0.1 = 50 cycles.
Key terms
- Physical vapor deposition. Deposition in which atoms travel from a solid source to the wafer
- Evaporation. Deposition by boiling a material in vacuum
- Sputtering. Deposition by knocking atoms from a target with ions
- Chemical vapor deposition. Deposition by a chemical reaction of gases on the wafer surface
- Plasma-enhanced CVD. CVD in which a plasma supplies energy so the wafer can stay cooler
- Epitaxy. Growth of a crystal film that continues the crystal of the wafer
- Atomic layer deposition. Deposition one atomic layer per cycle by two self-limiting reactions
- Precursor. A gas or vapor that supplies an element for the film
- Step coverage. The ratio of film thickness on a sidewall to that on the top surface
- Conformal film. A film with the same thickness on every surface
- Lift-off. Patterning by depositing a film over resist and then dissolving the resist
In the fab
Thin-film engineers work with furnaces, sputtering tools, and ALD reactors, and they measure every film they deposit. Many graduates start their careers in this area.
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).
Our Cleanroom Videos on the Training page include short videos on electron-beam deposition and atomic layer deposition.
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Created by Prof. Zafer Mutlu, ANNLab, University of Arizona.