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To make a chip, first of all, there must be light, how humans use the ethereal and invisible light to carve out a chip of the utmost delicacy, why photolithography is the basis of all processes in chip manufacturing, and the photolithography process in the fab, what are the specific steps, today this video, let me explain to you. After the first three video, we understand the process of making silicon wafers and the workings of the clean room, since the raw materials and clean room are already in place, then from this issue, we come to reveal each key link and equipment one by one, what is the use of mastering this knowledge? It is in case you accidentally travel to ancient times, the task of rebuilding modern civilization will be given to you! So don’t forget to take notes.
Principles of photolithography：
Silicon wafer to chip metamorphosis journey, let’s start with photolithography and photolithography machine. Lithography, as the name implies, is to use light to carve, why light? Because it is fast, for the 24-hour operation of the chip factory, time is money, speed determines the yield, and in the universe we live in, light is the limit of speed, snap, very quickly, ah, lithography is complete, that how the light is harnessed and used to carve out the chip? The answer is through photomasks, photolithography and photoresists.
The photomask is the blueprint of the chip, a glass mask with the layout of the integrated circuit engraved on it，
The photolithograph is like a nanoscale printer, emitting light to project the pattern on the photomask onto the silicon wafer，
Photoresist is a type of gel that can turn light and shadow into reality.
There are positive glue and negative glue, take positive glue as an example, it is a kind of light dead material, firm in the dark, single as long as the specific wavelength of light, it will be weak, and then can be dissolved and removed, negative glue is just the opposite, using this photosensitive photoresist, we can use light to engrave the chip.
For example, if we want to make a trench type memory chip (DRAM), we have to dig thousands of pits (Trench) on a flat silicon wafer to make a capacitor matrix. How can we dig so many pits with the same pitch and depth in a square millimeter at the same time through photolithography?
First, a layer of photoresist is applied to the silicon wafer in the dark, and then the photomask is illuminated for exposure, that is, the light is allowed to pass through according to the designed pit on the photomask and shine on the photoresist.This part of the gel will be weak, and then washed off by the solvent, and the remaining firm photoresist, it becomes a protective film，
Then just use the solvent that can corrode the silicon, the pit area without photoresist protection, corrosion off a layer, and finally remove the photoresist protective film, we will complete a large number of deep pits in the same time, the precise carving work, and this lithography after the silicon wafer directional subtractive corrosion, is etching (Etching),
In addition, we can also make pits by adding to the silicon wafer, for example, by passing a chemical gas, a layer of material grows evenly on the silicon wafer, at this time, the parts not protected by photoresist, the thickness increases, while the material that grows on the photoresist protective film will be removed together with the subsequent removal of the colloid, which is equivalent to the thickness remains unchanged, this directional additive method is deposition (Deposition),
etching and deposition, are important processes of the chip process, but also the two pits dug in this issue of the video, will be filled in detail later, so chivalry on the lithography, carved not the silicon wafer, but the silicon wafer on this layer of photoresist, carved photoresist as a blueprint, combined with the next step of etching or deposition, in order to carve the silicon wafer processing, in addition, in order to give electrical properties to the semiconductor silicon, we have to do ion implantation in specific areas, for which we also have to first lithography, do not want to inject examples of the region with photoresist film protection.
Because almost every etching, deposition and ion implantation requires lithography as a prerequisite, so in all the processes of chip manufacturing, lithography is the foundation, often occupying nearly half of the work hours and 1/3 of the cost of the entire process, which is why we first explain the reason for lithography, the process of lithography, summed up very simple, glue, exposure, rinsing, finished, yes, but not exactly.
Basic lithography process：
In actual production, every time we do lithography, we have to go through at least eight steps and three bakes. The first step is wafer cleaning and surface treatment, and the requirements of lithography for cleanliness far exceed those of the most advanced operating rooms. So before lithography, we have to give the silicon wafer a bath, wet cleaning first, then cleaning with deionized water to remove the pollutants adsorbed on the surface of the wafer, residues from the previous process, and impurities such as metal examples in the solvent, and then pass a gas called hexamethyldisilane, or HMDS for short, after this gas fumigation, the surface of the wafer will be fully dehydrated, because the surface hydrophilic hydroxyl groups are replaced with hydrophobic After this gas fumigation, the wafer surface will be fully dehydrated, because the surface hydrophilic hydroxyl groups are replaced with hydrophobic groups, so that the wafer can better adhere to the photoresist, so this step is also known as adhesion treatment，
After that, the filtered photoresist drops in the center of the silicon wafer, so that the wafer first high-speed rotation, the gel spread, and then rotate at a slower speed, so that the thickness of the gel is stable, this process, most of the photoresist, will be thrown out and wasted, leaving only a uniform layer of gel, related to the silicon wafer to paste a film, then ask what is the most afraid of film, yes, the most afraid of air bubbles, so the speed control of the applicator and the size of the exhaust air are Very careful, because once the photoresist bubbles, will affect the subsequent process caused by problems such as excessive etching, and another problem, I mentioned in the second phase of the video, the production of silicon wafers why the edge grinding (chamfering), photoresist in the spin coating, easy because of centrifugal force, accumulated at the edge, this accumulation of photoresist droplets on the edge, and even flow to the back, will affect the exposure of silicon wafers in the photolithography machine Therefore, after the coating is completed, the edge is removed (Edge Bead Removal), that is, the edge of the wafer is sprayed with solvent to remove a circle of photoresist at the edge.
After applying the glue, the wafer will be baked before exposure, also known as Soft Bake, this step, the robot arm will take out the wafer and put it in the oven or hot plate for baking, the purpose is to reduce the solvent content in the photoresist, so that it is thicker and stronger, to improve the stability of adhesion with the wafer, before baking is generally only about 100 degrees, “bake” a minute, the photoresist is not high-temperature resistant，
After the pre-bake is completed, the lithography machine comes on and exposes the silicon wafer，
In this video, due to space limitations, here is a three-sentence overview of what happens in a photolithography machine：
One, the photomask lens set and silicon wafer table will be precisely aligned and leveled; two, the light source to release light, three, move the workpiece table, so that the silicon wafer orderly exposure, three words, so that a lithography machine spent 1 billion, but do not underestimate this ordinary three words, which hides many pinnacles of human engineering, such as the lens, the surface undulation of no more than 0.05 nanometers Zeiss lens, is the closest Earth civilization to the three-body Droplet’s masterpiece, such as alignment, can synchronize the alignment and exposure of the dual bench system, is one of Asmac defeated the Japanese lithography manufacturers, such as the light source, driven by 40 kilowatts of extreme ultraviolet (EUV), is the liquid tin blast into a plasma when the dynamic light waves, after I will use two video to tell you in detail, why the lithography machine so expensive, which black technology used in the end，
Now back to our process, the silicon wafer from the photolithography machine out, but also to experience a post-exposure baking, referred to as after baking (PEB), the purpose of this step is to heat, so that the photoresist in the photochemical reaction, fully completed, can make up for the lack of exposure strength, but also to reduce the photocoel development, because of the standing wave effect, resulting in a circle of ripples, here do not understand no matter, later on the issue of photoresist, remember to come back to fill in the lesson，
After post-baking, the previously exposed part is dissolved and removed, and the graphics on the photomask are reproduced on the photoresist, which is developed and developed.
The usual method is to first use deionized water to wet the silicon wafer, and then the developer, generally an aqueous solution of tetramethylammonium hydroxide, evenly sprayed on the surface of the photoresist, so that the exposed part of the lithography, fully dissolved, and finally flushed away with deionized water, if the process is below 45 nanometers, because the size is too small, the specific surface area of the dissolved residues is too large, strong adhesion, can not be shaken off, can not be flushed away, so also spray nitrogen to blow them away，
After the development is completed, if it is a wet process, it needs to be baked again to make the protective film stiffer and more resistant to etching, so it is also called hard film baking.In order to further reduce the solvent content in the photoresist and prevent excess water from affecting the wet etching process afterwards, this step can also be omitted if it is followed by plasma etching, which is commonly known as the dry process.
Finally, we must use instruments to measure the film thickness of photoresist, the accuracy of the set of engraving, as well as the key dimensions, such as 7 nanometers, 5 nanometers, such as the process of line width defects, with an optical microscope is certainly not enough to see, you must use a higher precision, electron scanning microscope, or even atomic force microscope to do, the resolution of photoresist must meet the standard, in order to ensure that the etching, deposition or ion implantation of this blueprint, can be carried out smoothly, and a Silicon wafer, to repeatedly go through thousands of such operations, in order to carve out hundreds of millions of electronic devices and their corresponding circuit connections, and finally, become a small chip in our cell phones. The above is the basic principle of lithography and a brief lithography process.
Domestic substitution：
At present, the entire lithography process requires photolithography, photoresist and photomask, their degree of domestic substitution is very low, the difference is basically dependent on imports, or most of the dependence on imports, these key materials and equipment, we will analyze separately later, here only to talk about the relatively minor coating – baking – development of orbital all-in-one machine, as the name suggests, it is used with the photolithography machine, used to provide silicon wafer lithography in addition to exposure one-stop service, the price is not expensive, only 1/20 of the photolithography machine, currently in the domestic chip factory, coating baking development of all-in-one machine.
The latter, in particular, has a market share of more than 90%. As an important equipment supplier for chip manufacturing, Tokyo Electron is far less well-known than Asmac in the spotlight, but in the industry, it is a pivotal manufacturer, with equipment involved in various manufacturing processes such as photoresist coating, cleaning, deposition and dry etching, especially in the Currently the most advanced EUV process, photolithography is only Asmac can do, and photoresist coating only Tokyo Electronics can do.
Here, by the way, the Japanese semiconductor industry, once in the 80’s, as the day, but then in the U.S. technology sanctions and trade crackdown gradually withered, and Tokyo Electron is one of the very few survived, and further development and growth of enterprises, about the end of the last century, the rise and fall of Japanese semiconductors, is a very worthy of our current history. Back to the glue coating baking development machine, currently has the ability to replace the domestic only Shenyang’s core source micro, although the market share is now only about 4%, and due to technical and precision limitations, can only be used for more than 28nm process, but can break the monopoly of Japanese manufacturers to achieve the transformation from zero to one, for us to compete for technical discourse, improve bargaining chips, are very meaningful.
Do equipment and instruments, is a very difficult thing to earn money, but also requires a lot of talent and money, other countries decades of time to complete the accumulation of technology, it is difficult to catch up overnight, I explain each production process of the chip, will compare the gap between domestic and foreign, this is not to grow the ambition of others, on the contrary, as an engineer who has been in the factory, the flow of the chip, from time to time, the simulation software in English I hope more than anyone else that one day we will be able to use Chinese interface production equipment and measuring instruments, and I believe that this day will surely come, for this reason we must not be blindly confident and stagnant, but must first calmly and soberly realize where the gap is, in order to catch up with a down-to-earth attitude.
This is all the content of this video, of course, this issue in order to facilitate understanding, I skipped some knowledge and technical details, such as the need to do deep ultraviolet lithography (DUV) coated anti-reflection layer (BARC), water immersion lithography water barrier coating, and photomask distortion, photoresist poisoning and other issues, and like the process of lithography exposure, just a few seconds of things, the things inside I can The next issue I will talk about the evolution of lithography technology and the history of the war between lithography manufacturers, to see a lithography machine, what are some things, why sell him a billion, engineers and what crazy methods used, just barely let Moore’s Law lasted a few more years, and as a latecomer to Asmac, how in the 45nm technology line of battle, a battle to seal the deal, with Samsung TSMC, the chicken and the dog rose to heaven.
