Lithography Process and Equipment Principle

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Lithography Process and Equipment Principle

I. Introduction to the lithography process

The lithography process occupies an important position in the semiconductor manufacturing process, and the quality of lithography directly affects the performance of the device.

The following figure shows the main structure and distribution of ASML lithography machine, mainly including: light source, illumination system, mask table, mask transmission system, projection lithography objective lens, workpiece table, silicon wafer transfer system, alignment system, leveling and focusing system, environmental control system, machine frame and vibration reduction system, machine control system and machine software, etc., as shown in the following figure:

As shown in the figure below, the main steps of the lithography process include: cleaning the epitaxial wafer, homogenizing, pre-baking, photolithography, post-baking, development and film firming.

During the transportation of epitaxial wafers, they may be polluted by the environment, resulting in contaminants on the surface of epitaxial wafers. Therefore, the epitaxial wafer should be rinsed before homogenization to remove the contaminants on the surface of the epitaxial wafer, so as to avoid the introduction of impurities in the manufacturing process, which will affect the performance of the device. After cleaning, the epitaxial wafer is blown dry with a nitrogen gas gun or put on a hot plate to dry, and then the epitaxial wafer is placed in a homogenizer for homogenization.

In order to increase the adhesion between the photoresist and the epitaxial wafer and make the photoresist better attached to the epitaxial wafer, a layer of tackifier is generally applied to the epitaxial wafer before the photoresist is evenly coated. The homogenizer adsorbs the epitaxial wafer through a vacuum, and then rotates it at high speed so that the photoresist is evenly attached to the epitaxial wafer. The thickness of the photoresist is changed by adjusting the rotational speed and homogenization time.

Pre-baking is to put the epitaxial wafer on the hot plate for baking, and volatilize the excess solvent in the photoresist to solidify it, so that the photoresist can better adhere to the epitaxial wafer.

Lithography is to put the epitaxial wafer after pre-baking into the lithography machine, and use the exposure method to transfer the pattern on the mask plate to the photoresist on the surface of the epitaxial wafer to realize the transfer of the pattern.

In order to obtain a good lithography effect, it is necessary to explore the exposure dose (exposure time) and exposure focus (focus) corresponding to different graphics.

Generally, after photolithography, it is necessary to determine whether to post-bake the exposed epitaxial wafer according to the experimental conditions, if the photoresist is thick, post-baking will cause bubbles on the photoresist, which will eventually lead to lithography failure.

Post-baking diffuses the active material in the photoresist, eliminating the standing wave effect introduced by interference during the lithography process, and improving the lithography effect. Development is the chemical reaction of the developer and photoresist to dissolve the exposed part. In general, both exposed and unexposed parts react with the developer, so it is necessary to explore the development time. Immediately after the development is complete, the epitaxial wafer needs to be placed in a sink and rinsed with running deionized water to remove the developer remaining on the photoresist.

The firm film is the epitaxial wafer after baking and developing, so that the solvent in the photoresist volatilizes and increases the adhesion of the photoresist and the epitaxial wafer. After the film is solidified, a microscope is used to visually inspect the photolithography pattern to determine the lithography effect, and to determine whether to etch or re-lithography after degumming.

II. How the lithography machine works

According to the different exposure methods of the lithography machine, the lithography machine is divided into contact lithography machine, proximity lithography machine, projection lithography machine and step lithography machine, and the principle of different types of lithography will be introduced below.

As shown in Figure (a) above, the contact lithography machine is exposed through direct contact between the mask and the photoresist, and the structure is relatively simple, the light source is located at the focus of the lens, and the light emitted becomes a parallel light after passing through the lens, and all the graphics on the entire mask are transferred to the photoresist. This lithography machine has a resolution in the micron range, but the life of the mask is greatly reduced due to the direct contact between the mask and the photoresist.

The difference between the proximity lithography machine and the contact lithography machine is that the mask and the photoresist are not in direct contact, the mask is located at a distance of 10 μm from the photoresist, but the solvent volatilized by the photoresist will adhere to the mask, which will affect the service life of the mask. In addition, proximity lithography has a lower resolution than contact lithography and the exposure resolution is generally greater than 3 μm.

According to the movement mode of the lithography machine table, the projection lithography machine is divided into two types: step projection type and step scanning type.

As shown in Figure (c), a lens is added between the mask of the projection lithography machine and the epitaxial wafer to be exposed, so the mask is not contaminated by photoresist, and the lithography repeatability is better.

The size of the pattern exposed by the step-by-step projection lithography machine is the same as that on the mask, the ratio is 1:1, and the resolution of the exposure is about 1 μm.

The working principle of the step-by-step projection lithography machine is shown in the figure below:

The working method of the stepper projection lithography machine is shown in the following figure:

The difference between the step-by-step scanning projection lithography machine and the step-by-step projection lithography machine is that the ratio of the pattern on the mask and the size of the pattern exposed on the epitaxial wafer is 5:1 or 10:1, that is, the length and width of the pattern are reduced according to the ratio of 5:1 or 10:1, in which the ratio of 10:1 is used for exposure has a higher resolution, but the exposure time is 4 times that of the 5:1 ratio, so most of the compromised 5:1 ratio is used for exposure. The lithography machine of this scheme has the advantage of high resolution, which is generally less than 0.25 μm, which is the most widely used lithography machine at present.

Here's how a step-by-step scanning lithography machine works:

Compared with stepper projection lithography, the scanning lithography machine moves in a more complex way, and the lithography plate moves in the opposite direction while the table is moving.

This type of motion increases the lithography area.

How can I learn more about the differences between step projection and step scan movements? The figure below shows the difference between the two working methods and scanning area, the step scan motion mode has a larger working area and a more precise locality.

If the two working methods are taken as an example, the step projection is more like a rectangular funnel, in lithography, the light in the rectangular funnel is ejected at the same time, and the lithography work is completed in one go. A step-by-step scanning lithography machine is like a conical funnel, and while the lithography is working, the light fills down and the table moves. The light on the workbench is more uniform and delicate.

III. Positive and negative glues for photoresists

Lithography technology mainly transfers the pattern on the mask to the photoresist through the exposure of the lithography machine and the development of the exposed epitaxial wafer. The main components of photoresist are resins, photoinitiators and solvents. After exposure, the chemical properties of the photoinitiator in the photoresist composition are changed, and the pattern transfer is completed after development. Photoresists are divided into positive photoresists and negative photoresists according to their properties after exposure development. As shown in Figure (a) below, positive photoresist is removed from the exposed area after development, and the commonly used positive photoresist is SPR200 produced by DuPont; In contrast to positive photoresists, as shown in Figure (b), negative photoresists are the photoresists in the exposed area that are retained after development, and the commonly used negative photoresists are N244, etc.

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