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Detailed Explanation of Chip Manufacturing. Photolithography Technology and Basic Process. Study Notes 4
1. Introduction
To make a chip, you must first have light. How human beings carve an extremely delicate chip with ethereal and invisible light. Why lithography is the basis of all processes in the chip manufacturing process. As for the lithography process in the fab, what are the specific steps?
2. Photolithography
Lithography : Engraving with light.
Why light?
Answer: Because it is fast. For a 24-hour chip factory, time is money, and speed determines output. In the universe we live in, light is the limit of speed.
So how do we harness light to carve chips?
Answer: Through the photomask (MASK) , photolithography machine , and photoresist .
(1). Photomask
Photomask : It is the blueprint of the chip, a glass light-shielding plate engraved with the layout of the integrated circuit.
(2). Photolithography machine
Lithography machine : Like a nanoscale printer, it projects the pattern on the photomask onto the silicon wafer by emitting light.
(3).Photoresist
Photoresist : It is a colloid that turns light and shadow into reality, and can be divided into positive and negative resists.
Positive glue: It is a material that dies when it sees light. It is strong in the dark, but it will weaken as long as it is irradiated by light of a specific wavelength. And then can be dissolved and removed.
Negative glue: just the opposite.
By taking advantage of the photosensitivity of photoresist, we can use light to engrave chips.
(4). Tunnel digging
For example, if we want to make a trench memory chip (DRAM), we have to dig thousands of trenches (Trench) on a flat chip to make a capacitor matrix.
So how to dig a tunnel for the chip?
4.1 Resist
Resist : Covertly apply a layer of photoresist to the silicon wafer.
4.2 Thin coat of resist
Thin coat of resist: apply a thin layer
4.3 Patterned mask
Patterned mask : Then illuminate the photomask for exposure
, allowing the light to pass through the designed pits on the photomask
4.4 Chemical reaction
The part of the colloid that is irradiated becomes weak and is washed away by the solvent
4.5 Developing
The remaining firm photoresist forms a protective film.
4.6 Organic solvent
Organic solvent : organic solvent, through a solvent that can corrode silicon, etch away a layer of the pit area that is not protected by photoresist, and finally remove the photoresist protective film.
4.7 Etching
Etching :
In this way, we can complete the precise carving of a large number of deep pits at the same time. After this kind of engraving, the subtractive etching of the orientation of the silicon wafer is called etching.
4.8 Deposition
Deposition : Deposition, contrary to the subtraction of etching orientation, deposition is a process of adding orientation to silicon wafers.
Lift-off Technique
Lift-off Technique : Lift-off technique
In addition, we can also do additives to the silicon wafer to make pits, such as passing through chemical gases to uniformly grow a layer of material on the silicon wafer. At this time, the thickness of the parts that are not protected by the photoresist will increase, and the substances that grow on the photoresist protective film will be cleaned together with the subsequent removal of the colloid, which means that the thickness remains unchanged.
This kind of directional addition is deposition (Deposition)
3. Lithography in a narrow sense
In the narrow sense , photolithography does not engrave silicon wafers, but the layer of photoresist on the silicon wafers.
4. Ion implantation
The etched photoresist is used as a blueprint, and then combined with the next step of etching or deposition. In order to engrave and process the silicon wafer, in addition, in order to give electrical characteristics to the semiconductor silicon, we also need to do ion implantation in specific areas. For this reason, photolithography must be carried out first, and the areas where ions do not want to be implanted are protected with a photoresist film.
Five, the status of lithography
Just because almost every etching, deposition and ion implantation requires photolithography as a prerequisite , photolithography is the foundation of all chip manufacturing processes . It often occupies nearly half of the working hours of the whole process. and 1/3 of the cost.
6. Detailed process of lithography
In summary, the photolithography process is:
coating-exposure-washing
In detail, there are eight steps and three times of baking
1. Cleaning and surface treatment wafer clean and vapor prime
Silicon wafer cleaning and surface pretreatment (wet and deionized water cleaning), the cleanliness requirements of lithography far exceed the state-of-the-art operating room.
Therefore, before photolithography, the chip should be bathed, wet cleaned first, and then cleaned with deionized water. To remove pollutants adsorbed on the surface of silicon wafers. Residues from the previous process, and impurities such as metal ions in the solvent.
After that, a gas called hexamethyldisilazane, referred to as HMDS, is introduced. After gas fumigation, the surface of the silicon wafer will be fully dehydrated, because the surface hydrophilic hydroxyl group is replaced by a hydrophobic group, so that the silicon wafer will It can better adhere to the photoresist, so this step is also called adhesion enhancement treatment .
2. Spin coat
Drop the photoresist on the center of the silicon wafer, let the silicon wafer rotate at high speed first, spread out the colloid, and then rotate at a slower speed to stabilize the thickness of the colloid. In this process, most of the photoresist will be thrown out and wasted, leaving only a uniform layer of colloid, which is equivalent to pasting a film on the silicon wafer.
We all know that film sticking is afraid of air bubbles, so the speed control and exhaust size of the gluing machine are very particular, because once air bubbles are generated in the photoresist, it will affect the subsequent process. Causes problems such as overetching.
Another problem is that the production of silicon wafers requires grinding (chamfering), and the photoresist is easy to accumulate at the edge due to centrifugal force during spin coating. The photoresist droplets accumulated on the edge or even flow to the back will cause Affects the exposure of the silicon wafer in the lithography machine. Therefore, after the glue coating is completed, Edge Bead Removal is required: that is, spray solvent on the edge of the silicon wafer to remove the photoresist around the edge.
3. Soft bake
After gluing, the silicon wafer should be baked before exposure.
In this step, the robotic arm will take out the silicon wafer and bake it in an oven or hot plate to reduce the solvent content in the photoresist. Make it thicker and stronger, and improve the stability of adhesion to the silicon wafer. Generally, the pre-baking temperature is only about 100 degrees, and it is "baked" for one minute. The photoresist is not resistant to high temperatures .
4. Alignment and exposure alignment and exposure
After the pre-baking is completed, the photolithography machine is put on the field to expose the silicon wafer.
Mainly the following things happened:
1. The photomask, lens group (Zeiss) and wafer stage are precisely aligned and leveled.
2. The light source emits light, extreme ultraviolet light (EUV) driven by a 40kW laser
3. Move the workpiece table to expose the silicon wafers in an orderly manner
These three processes cost 1 billion yuan.
5. Post exposure bake
Abbreviated as PEB.
The purpose of this step is to fully complete the photochemical reaction in the photoresist by heating, which can make up for the problem of insufficient exposure intensity, and at the same time reduce the circle of lines caused by the standing wave effect after the photoresist is developed.
6. Develop and rinse develop and rinse
After post-baking, the previously exposed part is dissolved and removed, and the pattern on the photomask is reproduced on the photoresist.
The usual practice is to wet the silicon wafer with deionized water first, and then spray the developer (an aqueous solution of tetramethylammonium hydroxide evenly on the surface of the photoresist) to fully dissolve the exposed part in the photolithography. , and finally rinsed away with deionized water. If it is a process below 45nm, because the size is too small, the specific surface area of the dissolved residue is too large, the adhesion is strong, and it cannot be shaken off or washed away, so it is necessary to spray nitrogen to blow them away.
7. Hard bake
Make the protective film stronger and more resistant to etching. In this way, the solvent content in the photoresist is reduced, and excess moisture is prevented from affecting subsequent wet etching.
8. Measurement detection inspection
Finally, an instrument is used to measure the film thickness of the photoresist. An electron microscope or even an atomic force microscope is generally used. The resolution of the photoresist must meet the standard to ensure that the etching, deposition or ion implantation based on this blueprint can be carried out smoothly.
7. Photolithography equipment
Gluing-baking-developing orbital integrated machine
It is used in conjunction with a photolithography machine to provide silicon wafers with one-stop photolithography services other than exposure. The price is not expensive, only 1/20 of the lithography machine.
At present, it is basically monopolized by Japan's SCREEN and Tokyo Electronics (TEL, accounting for 90% of the market).
As an important equipment supplier for chip manufacturing, Tokyo Electronics is far less well-known than ASML under the spotlight. But in the industry, it matters. The equipment is involved in various manufacturing processes such as photoresist, cleaning, deposition and dry etching. Especially in the current state-of-the-art EUV process. The photolithography machine can only be done by ASML, and the photoresist coating can only be done by Tokyo Electron.
At present, only Xinyuanwei in Shenyang has the ability to replace the gluing-baking-developing orbital all-in-one machine . The market share is only about 4% at present, and it can only be used in the process above 28nm. But being able to break the monopoly position of Japanese manufacturers and realize the transformation from 0 to 1 is of great significance for us to compete for the right to speak in technology and increase the bargaining chip in negotiations.