Etching
Etching is the process of removing layers of unwanted silicon, silicon dioxide, aluminum, and other materials from a wafer. This occurs after a wafer has a layer of photoresist which has been exposed to uv light through a mask.

This are two types of etching, wet and dry.

The trend in VLSI is toward dry etching because it provides better etching uniformity than wet etching. That is the layers of an wanted silicon and other materials are removed at a uniform rate across the wafer.

Wet etching involves the use of nitric and hydrofluoric acid. The nitric acid forms oxide in the silicon and hydrofluoric acid dissolves the oxide. The most important part of etching is to control how much and how deep the material, silicon, silicon dioxide, and doped silicon is etched away. To control the etching process, the following parameters are used:

1) Concentration: How concentrated the acid is, the ratio of hydrofluoric acid to ammonium fluoride can vary between 4:1 to 10:1. The ratio used will depend on the amount and type of dopant in the silicon and the etching rate wanted.

2) Time: How long the acid is used to etch. This really depends on the etch rate of the acid. Typical etching time is between 30 to 60 seconds.

3) Temperature: At what temperature to etch the silicon. This is the most critical factor, typically the higher the temperature, the faster the etching rate becomes.

4) Agitation: The delivery method of the acid. Some methods include: thermal convection, spray, mechanical, bubble, and ultrasonic. Each method has its advantages and disadvantages. The spray method is considered the best because it produces the least amount of undercutting, cutting underneath the photoresist, and the highest etch rate.

Other materials that are etch include Silicon nitride and aluminum.

Silicon nitride is wet etched using either phosphoric acid or hydrofluoric acid. The same factors listed above for etching silicon also apply to silicon nitride.

Aluminum, used to connect the transistors in a chip, is etched using solutions consisting of 80% phosphoric acid, 5% nitric acid, 5% acetic acid, and 10% water. The etching solution also can include silicon and copper to ensure that a good contact is formed between the aluminum paths and silicon transistors.

Dry etching

Dry etching involves using hot plasma gas to remove the silicon and other materials. The gas contains one or more halogenated hydrocarbons atoms and either fluorine, chlorine, bromine, or iodine. This gas reacts with the semiconductor layers of silicon, silicon oxide, and metal to form volatile compound byproducts. The byproducts are absorbed by the etch chamber. Unlike wet etching, dry etching does not remove a lot of the photoresist as it is etching.

In a plasma reaction, the etching process is controlled by two factors: the type of plasma discharge (inert or reactive) and the amount of bombardment.

There are four types of reactive gases used to perform etching, volatile gas discharges and in-volatile gas discharges. Both can use either low or high energy ions.

There are four different methods to deliver the plasma:

1) plasma etching

2) sputtering etching

3) reactive ion beam etching

4) reactive ion etching

Plasma etching consists of: a plasma chamber, where the wafer is placed; a radio frequency power source, this helps the plasma to breakdown the silicon; a gas flow control system, to deliver the plasma; and a vacuum system to remove the waste gas.

Sputtering etching consists of turning a heavy noble gas such as argon into plasma and bombarding a wafer with the gas.

Reactive ion beam etching consists of directing high energy ions against the wafer to perform the etching. This etching process relays more on the momentum of the gas particles rather than a chemical reaction to perform the etching.

Reactive ion etching also consists of directing high energy ions against the wafer to perform the etching. This etching process relays more on chemical reaction to perform the etching.

1) Repeatability: being able to reproduce the mask details accurately in the silicon.

2) Uniformity: Etch uniformity across the wafer must be consistent.

3) Selectivity: Etch Contrast across the wafer must be high, typically 7:1 to 30:1.

4) Slope angle control: The slope of the holes created for etching from 0 to 45 degrees must be accurate to allow for later stages of fabrication.

5) Operator and Environment Safety: The etching process should have operator safety, plant safety, and environmental safety regulations.

6) Radiation damage: The radiation used in etching must not damage the devices created.

7) Thermal distortion: The heat from etching must not damage the silicon in the wafer.

8) Throughput and capital expense: The wafer throughput must be high enough to satisfy the economic requirements and production demands.

9) Cleanliness: The etching equipment must not contaminate the wafer.

10) Mask removal: The photoresist mask must be completely and easily removed

Dry Etching

PolysiliconNitride Dry Etcher (Click on the image for a better view!)

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Ion Implantation