Chapter 525 Resolution 10μm
The Cifei Factory has made up its mind, and President Xiao has traveled to Beijing, but Gao Zhendong's affairs are far from over.

At this time, he was taking the information about another Fifth Master - the bomber - sent by the Defense Work Committee, and calculating and considering it carefully.

What he wants to leverage is not just the radar installation on the Gong-5.

Just when he was concentrating on calculating, there was a knock on the door.

Gao Zhendong stood up and opened the door. At the door were Yi Zhonghai and his junior sister Ma Juan from Wudaokou. Although Ma Juan had a straightforward personality and some masculine heroic qualities, she was very thoughtful and was just the right person to represent the electronic laboratory of the branch factory to follow up on the assembly of the lithography machine.

"Senior Brother Gao, good news, good news!" Ma Juan was not as formal as Wu Shengyuan. She also called him Senior Brother Gao at work, which made Gao Zhendong feel very happy.

"Has the lithography machine been assembled?" She came here with Yi Zhonghai, and there couldn't be anything else except this.

"Yes, it's installed. We tried it first, and all the indicators are fully up to standard."

"Great, let's go and take a look." Gao Zhendong put away the things in his hands, stood up and walked towards the lithography machine trial production workshop.

Yi Zhonghai and Ma Juan followed behind. Ma Juan smiled and said, "The reason we came here is to invite you to preside over the final overlay accuracy test."

Gao Zhendong nodded repeatedly: "Yes, let's try. Let's go! Let's go!"

As for other indicators, it can be said that Yi Zhonghai and his master chefs, with the cooperation of Ma Juan, basically completed them all. The overlay accuracy is the final comprehensive test.

Walking into the trial production workshop of the lithography machine and looking through the glass into the clean room, Gao Zhendong saw something that looked like a ventilated test cabinet in a chemical laboratory, or a microscope device with biological protection used in biological research. It was impossible to tell what the other items were used for, but the microscope was very conspicuous.

This microscope is used for manual alignment and inspection, mainly to solve the alignment problem during overlay. Gao Zhendong designed alignment marks on the mask, but the alignment operation is manually performed on this lithography machine, and the microscope is used to do these things.

In theory, Gao Zhendong has seen this machine many times, with many people coming and going and making modifications. He can be considered an old friend of his, but when he saw it this time, he still felt excited.

Finally there is a result. In Gao Zhendong's mind, this thing is no longer just a piece of equipment, but a spirit.

In his previous life, Gao Zhendong had been in online confrontations for many years, and his opponents' criticisms gradually evolved from being able to criticize whatever they could get their hands on to being able to criticize whatever they could.

Finally, I simply became a nail house, squatting in those few areas and refusing to move. No matter which direction the wind blew, I remained unmoved, and just kept saying, "Yes, yes!... Do you know how many types of light sources there are in a lithography machine?" Gao Zhendong became more and more impatient as he watched, and walked away with the keyboard.

Although he had traveled far, these things were indeed something he had not yet completely acquired until he traveled through time. At least, he had not acquired the high-end ones, and the photolithography machine was one of them.

Damn it, I'm going to build one myself for fun in this life. I don't know what kinds of light sources there are, but I know this one works, and it's pretty good!
Gao Zhendong also put on protective clothing and walked into the clean room with several test personnel.

The test personnel had already prepared for the test. After they came in, they turned on a few switches and signaled to Gao Zhendong: "Mr. Gao, it's ready."

Gao Zhendong picked up a special clamp, carefully picked up a single-crystal silicon wafer from the silicon wafer rack with desiccant, placed it on the workbench, and then flipped the switch, and the silicon wafer was adsorbed and fixed.

These single crystal silicon wafers have been cleaned, dried and other processes, and their adhesion to the photoresist is the greatest at this time.

Then Gao Zhendong took a step back and handed the next experiment to Ma Juan.

What he wants is the feeling of satisfaction at the beginning. As for the specific operations, these testers are much more skillful than him. Professional matters should be left to professionals.

To say that Gao Zhendong's lithography machine is primitive is truly primitive. After fixing the silicon wafer, Ma Juan turned on the switch and the silicon wafer began to rotate.

This is Gao Zhendong's design, which is done in one step. For the coating of photoresist, he directly used the spin coating method. He didn't know what coating method our earliest photolithography machines used in the past, but he knew that spin coating definitely had the best overall performance.

The spin coating method is used on a flat and simple surface such as a single crystal silicon wafer. It has strong flattening ability, easy to control thickness, high coating density, and uniform thickness. It is not without defects, but it is acceptable and processable.

The rotation quickly reaches the preset speed. When the coating material is determined, the coating thickness and other coating result parameters are highly related to the rotation speed. While rotating, the workpiece table is also heating the silicon wafer to further remove the water vapor that has re-attached to the surface.

These small details were brought over by Gao Zhendong from his previous life. They are inconspicuous, but very important.

After the heating time met the preset requirements and the rotation had already stabilized, Ma Juan picked up the dropper. Yes, it was the kind of dropper used in chemical experiments, not fancy at all, but it was enough for Gao Zhendong's photolithography machine.

Droppers are naturally not very precise in their measuring function. For example, one milliliter of water is about 15 drops. In short, when the viscosity of the liquid is determined, the volume of each drop of liquid dripping from the dropper is basically the same. The same is true for changing droppers, as long as it is a normal dropper.

The excess photoresist will be thrown away by the rotating silicon wafer, which is also the advantage of spin coating.

Ma Juan dripped some photoresist into the center of the rotating silicon wafer. The photoresist quickly spread along the surface of the silicon wafer as it rotated, very evenly, as if it did not exist.

This is also due to the good adhesion between photoresist and silicon wafer. Otherwise, before applying photoresist, a substrate must be applied to strengthen the adhesion of photoresist.

The silicon wafer slowly stops rotating, and the worktable starts to heat the silicon wafer again, which is curing the photoresist.

After curing was completed, Ma Juan began to press buttons to adjust the worktable and place the silicon wafer in the appropriate position under the optical system.

Although Gao Zhendong asked the comrades at the Northeast Institute of Optics to achieve a projection range of 160mm diameter, considering the yield rate of crystal pulling and the fact that the maximum distortion of the optical system occurs at the outermost edge, the specification of the silicon wafer is now set at 130mm, which is about 5 inches. However, this is our native process, so we naturally do not use inches as the unit of measurement for silicon wafers.

As the earliest and most primitive lithography machine, Gao Zhendong did not use the most primitive contact lithography. In this method, the silicon wafer and the mask are in direct contact, and the resolution is higher than the proximity type.

However, for contact lithography, the photoresist or individual dust on the silicon wafer will contaminate and damage the mask. It can be used for research but not for mass production.

He skipped contact lithography and used proximity lithography. During actual lithography, there was an extremely small distance between the silicon wafer and the mask, on the order of 10μm, so that damage to the mask could be avoided.

As for the downside, the resolution of proximity lithography is worse than that of contact lithography. In the best case, it is about 2μm, which is enough for Gao Zhendong now.

When using proximity lithography, the mask and the workpiece are separated, which is beneficial for automated lithography. This is why Gao Zhendong puts the mask into the optical system. The mask basically does not move in the optical system, and it is the workpiece table that moves.

After leveling, the worktable controls the silicon wafer to approach the mask, and finally reaches the distance set in the design - 20μm.

You don't need to align too carefully this time because it is the first time engraving. Strict alignment is a matter of overlay engraving. If it is overlay engraving, you also need to align the overlay marks.

The next step is exposure and development, which sounds similar to film photography, and in fact the principles are basically the same.

After the development was completed, Ma Juan began to check the image quality with a microscope. It took a long time and her eyes were probably tired. Finally, she raised her head and nodded to Gao Zhendong with a happy face.

The mask this time is a standard mask for process testing. Judging from its appearance, the resolution of at least 10μm should be no problem.

"Brother Gao, it's the same as our previous test. There is no problem with the line width of 10μm. It's very stable."

If there is a problem with the image quality, you have to take out the silicon wafer, wash off the photoresist, and start over.

Silicon wafers are very expensive these days and cannot be wasted. Even if silicon wafers are not that expensive decades later, they still need to be cleaned. If the problem lies with the photolithography steps in the middle and late stages of the process, the previous processes are also expensive!
Gao Zhendong was excited and nodded to her, indicating that she should continue.

Next, according to the normal integrated circuit process, it should be sent to other processes, and depending on the purpose of the lithography, such as etching, doping, ion implantation, metal removal, etc. should be carried out. However, this is to test the resolution and overlay accuracy of the lithography machine. This process is not necessary and cannot be done because those things are not available here.

For example, etching looks simple, all you need to do is corrode it, but in fact, it relies on a whole special safety management line behind it. It will be troublesome to build one here, as it is not used often.

Not to mention the mixed processes that require equipment, it is impossible to do.

Considering that after development, only the required part of the photoresist is left, the experimenters' idea is to apply photoresist again to see the degree of overlap between the two photolithography and development results.

This is also a stupid method. If in this case, the residues of the old photolithography process affect the adhesion of the new photoresist, resulting in poor test results, then consider the test process of photolithography - 1274 etching - photolithography.

The line width alone is 10μm. In fact, the process of the lithography machine cannot reach 10μm, because there are other parameters that determine the process of the lithography machine. One of the most important ones is the overlay accuracy, which is exactly what Gao Zhendong will test today.

Whenever integrated circuits are made, even if the process is as simple as PMOS, overlay etching is required, because one photolithography cannot even produce a transistor, let alone form a complete circuit.

(End of this chapter)