How to Engrave a Chip: Etching Principle |Chip Manufacturing Detail 05

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What is the limit of silicon wafers to be engraved, but a layer of glue, after which this graphic layer of photoresist has to be used as a template to engrave the various structures on the chip, what is the means of using chemistry or physics after this photolithography? Join us in having the money and time to line up an atom in the shape of love you with a scanning tunneling microscope probe in the ultra-low temperature environment of the laboratory. But if you want both the speed of industrial mass production and the precision at the microscopic level, the only way to do it is to lithograph and then etch.

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Today about the chip weight loss and thinning, last quarter we explained the basic process of photolithography, that is, the silicon wafer spin coated with a layer of gel, and then after the photolithography machine exposure, and the development of the development of the process, the integrated circuit plate printed on the photoresist, so the photolithography machine engraved not, the process of refinement of the silicon wafer body, it is called etching Etching.

Wet etching:

Etching techniques can be divided into two categories: Wet etch, which uses liquid chemicals to dose silicon wafers in a bathing shower of strong acids and bases, and dry etch, which uses gas plasma to partially slim down silicon wafers under the ion bombardment of chemical gases.

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For example, in the Middle Ages, the Europeans would use acidic solutions to etch carvings on metal armour, and in the Ming dynasty’s “Ge Gu Yao Lun”, it was also recorded that where swords and knives were polished and cleaned, they were alummed with gold silk alum, and their flowers (patterns) were visible. The Ming dynasty people said that gold silk alum is a kind of sulphate, so when history entered the era of integrated circuits, the first thing that engineers thought of was also to use the corrosive nature of various acids and bases to carve out chips on a large scale, and the name of etching in the process technology was thus created.

However, unlike armour and blades, there are not only metals on chips, but also silicon, silicon oxide, silicon carbide, gallium nitride and various other media, so it is much more difficult to carve on them, for example, silicon as a semiconductor is more stable than metal, and it does not eat strong acids in general.

This mixture of three acids should never be used to dip dumplings. The hydrofluoric acid in it is extremely dangerous, and when it comes into contact with the skin, it will quickly penetrate into the tissues and erode the bones, which is why hydrofluoric acid has a more resounding common name in folklore – “bone water”. In case of inadvertent contact with high concentrations of hydrofluoric acid in etching, you must rinse a lot and go to the hospital for emergency treatment, lightly pull out your nails, or amputate your limbs, this is not in the torture, this is in the clean poison to save lives, so hand-rubbed chip work, or left to manufacturers to do it, the chip factory in the wet etching, is not as long as the silicon wafer after photolithography soaked in the acid bath, sit and wait for the extinction of photoresist The parts that are protected by photoresist will be eroded away and that’s it? It may look that way, but it’s not that simple.

Etching indicators:

There are three key indicators in etching: etch rate, selectivity and directionality, where etch rate refers to the rate at which a substance is dissolved, usually measured in microns of thickness lost per minute. In the chip process, the depth of etching of a part is usually controlled by the etch time, so it is important to ensure a precise and constant etch rate, which requires a high degree of purity, proportionality and concentration variation of the etchant.

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For example, the previously mentioned silicon etching “triacid bone water”, the name of the industry is HNA solvent, etching rate of up to 480 microns per minute, a silicon wafer put down for two minutes, even the usual favorite fishing I can not get up, so the ratio of etching solution is very delicate, according to the needs of the process to make adjustments, such as increasing the proportion of nitric acid in HNA, help the surface fully oxidized and then corroded, the etched silicon wafer will be smoother, and conversely increase the proportion of hydrofluoric acid, etching faster, the surface will be more rough;

Secondly, temperature and stirring will also affect the etching rate, in our etching acid bath, we will be equipped with heating and stirring functions, after all, wet etching is essentially a chemical reaction, very sensitive to temperature, a temperature difference of 5 degrees is enough to cut the rate in half, so the temperature control in production is very important, otherwise the same etching process, the silicon wafer in the northeast may be out of the pot, the silicon wafer in Sichuan is missing,

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In addition, in order to make the chemical etching more violent, sometimes more physical buffs are applied, such as the use of ultrasonic devices to generate mechanical waves to shake the etching solution, so that the liquid is continuously compressed and released in the uninterrupted high-frequency vibration, resulting in a large number of tiny bubbles, which is called cavitation.

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A common example is the ship’s propeller, when the paddle rotates at high speed underwater, it will be attacked by bubbles and wear, because the generation and rupture of these bubbles, will release energy, but in the chip process, control the amplitude and frequency of ultrasound, you can use the appropriate diameter of the bubble, in the etching to take away the impurities on the surface of the silicon wafer, for example, when we use etching solution to clean the more sophisticated chips, you can put High-frequency ultrasound, replaced by a smaller amplitude megasonic frequency (Megasonic), to weaken the cavity and resonance effect, to avoid damage to the structure of the smile device, note that the cleaning mentioned here, not to take ultra-pure water to the silicon wafer simple shower, but the use of etching solution corrosion effect, wash off a layer of silicon wafer skin, to remove surface impurities, the surface defects (defect),

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This is also the most widespread application of wet etching, wet cleaning, but the real etching, the focus is not “etching”, but “engraving”, in order to follow the template of photoresist, to the silicon wafer to do fixed-point directional engraving, that etching in addition to the etching rate, there are two important indicators, the selection ratio and directionality,

Assuming the selection ratio of silicon to photoresist is 10:1, then for every 10 microns of silicon carved off, 1 micro-thickness of photoresist will be lost, so the higher the selection ratio, the less effort and safer the etching will be, for example, in the later stage of metal etching, we want the selection ratio to be as high as possible, so that only the metal is etched and not over etched into the lower layer, affecting the silicon or oxide structure already done,

Anisotropic:

In addition to the selection ratio, the direction of etching is more important. Generally, wet etching has no direction, or etching is uniform in all directions, which is called isotropic.

When we design a chip, the ideal situation is to have no lateral corrosion, so that the etching takes place in only one direction, which is called anisotropy, so how can we achieve anisotropy in wet etching? This is where the various acidic chemical bone waters don’t work well and engineers have started to use alkaline metal solvents, such as potassium hydroxide, which can significantly reduce lateral corrosion and achieve a degree of anisotropy,

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The principle is to use single crystal silicon in different crystal upward, the corrosion resistance of potassium hydroxide is different, the solution in the bottom plane corrosion rate is hundreds of times stronger than in the side bevel, so in the early days of semiconductor production, this kind of alkali metal solvent, was used to do device modeling, such as longitudinal carved out the power transistor V groove, but the disadvantage of alkali metal solvent is obvious, is that the metal ions in it, will affect the threshold voltage during, more complex chip carved a few times will fail, so engineers developed a variety of metal-free etching solution, such as TMAH solvent, EDP solvent, hydrazine solvent,

The characteristics are safe for chips, but for humans, one is more dangerous than the other, briefly introduce these several etching solvents. The safest one is TMAH (Tetramethy ammonium hydroxide), which is more beneficial, but a bit more toxic, the main ingredient is tetramethylammonium hydroxide, which is a neurotoxin and can be lethal on contact.,

In addition to etching, TMAH is also often used as a developer after lithography, which is widely used in factories and laboratories; then there is EDP solvent, which is scientifically known as ethylenediamine catechol, where ethylenediamine is a poison and catechol is another poison, both of which are common ingredients of pesticides.,

Finally, the ultimate weapon in the chip factory, hydrazine, which is not only highly toxic, but also flammable and explosive. Before it was used to etch chips, it was often used as launch fuel for rockets, so wet process engineers will tell you: it is not only love that makes people suffer, but also Etching, and if you don’t operate properly, you will be poisoned by hydrofluoric acid, or you will be ignited by hydrazine and launched into the sky.

That introduced so many useful reagents, but in the current mainstream front-end process, they are basically eliminated, because in the final analysis, the chemical reaction-based wet etching, can only minimize the lateral etching, complete anisotropy it can not do, to the early stages of the chip process, the device structure is relatively rough, line width spacing is also large enough, etching directional poor, the problem is not big, but with the process into the sub-micron Even for relatively rough memory chips such as DRAM, to carve out the narrow and deep capacitance grooves inside requires a highly directional carving method, which is dry etching.,

Dry etching:

The wet process uses liquids, while the dry process uses gases. Chip factories consume many special gases every day, a large proportion of which is used for etching, and these gases, after being precisely proportioned, are passed into the reaction chamber and then fully or partially ionised by means of capacitive or inductive coupling to form a plasma or ion beam, which is accelerated by an electric field and shot at the silicon wafer for etching, a form of engraving with both physical and chemical properties,

If the focus is on chemical attack, you can pass more fluorocarbon gas, sacrificing some directionality to achieve a better selection ratio, if the focus is on physical attack, you can pass more inert gas, such as argon, mainly with high-energy particles, sputtering silicon to ensure longitudinal etching, but the disadvantage is that the selection is relatively low, easy to be indiscriminate, the upper and lower layers penetrate together, so dry etching equipment will generally be equipped with end-point detection function, try to avoid over-etching, or more safely, we first deposited an additional layer of silicon nitride or other substances on the chip, used as a stop layer,

Deposition, mentioned here, is the third important process in chip manufacturing, in addition to photolithography and etching, and the three are often interrelated, so I will wait to explain to you more details about dry etching and the more advanced atomic layer etching (ALE) technique until I have first explained the principles of deposition.

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The principle is to use single crystal silicon in different crystal upward, the corrosion resistance of potassium hydroxide is different, the solution in the bottom plane corrosion rate is hundreds of times stronger than in the side bevel, so in the early days of semiconductor production, this kind of alkali metal solvent, was used to do device modeling, such as longitudinal carved out the power transistor V groove, but the disadvantage of alkali metal solvent is obvious, is that the metal ions in it, will affect the threshold voltage during, more complex chip carved a few times will fail, so engineers developed a variety of metal-free etching solution, such as TMAH solvent, EDP solvent, hydrazine solvent,

The characteristics are safe for chips, but for humans, one is more dangerous than the other, briefly introduce these several etching solvents. The safest one is TMAH (Tetramethy ammonium hydroxide), which is more beneficial, but a bit more toxic, the main ingredient is tetramethylammonium hydroxide, which is a neurotoxin and can be lethal on contact.,

In addition to etching, TMAH is also often used as a developer after lithography, which is widely used in factories and laboratories; then there is EDP solvent, which is scientifically known as ethylenediamine catechol, where ethylenediamine is a poison and catechol is another poison, both of which are common ingredients of pesticides.,

Finally, the ultimate weapon in the chip factory, hydrazine, which is not only highly toxic, but also flammable and explosive. Before it was used to etch chips, it was often used as launch fuel for rockets, so wet process engineers will tell you: it is not only love that makes people suffer, but also Etching, and if you don’t operate properly, you will be poisoned by hydrofluoric acid, or you will be ignited by hydrazine and launched into the sky.

That introduced so many useful reagents, but in the current mainstream front-end process, they are basically eliminated, because in the final analysis, the chemical reaction-based wet etching, can only minimize the lateral etching, complete anisotropy it can not do, to the early stages of the chip process, the device structure is relatively rough, line width spacing is also large enough, etching directional poor, the problem is not big, but with the process into the sub-micron Even for relatively rough memory chips such as DRAM, to carve out the narrow and deep capacitance grooves inside requires a highly directional carving method, which is dry etching.,

Dry etching:

The wet process uses liquids, while the dry process uses gases. Chip factories consume many special gases every day, a large proportion of which is used for etching, and these gases, after being precisely proportioned, are passed into the reaction chamber and then fully or partially ionised by means of capacitive or inductive coupling to form a plasma or ion beam, which is accelerated by an electric field and shot at the silicon wafer for etching, a form of engraving with both physical and chemical properties,

If the focus is on chemical attack, you can pass more fluorocarbon gas, sacrificing some directionality to achieve a better selection ratio, if the focus is on physical attack, you can pass more inert gas, such as argon, mainly with high-energy particles, sputtering silicon to ensure longitudinal etching, but the disadvantage is that the selection is relatively low, easy to be indiscriminate, the upper and lower layers penetrate together, so dry etching equipment will generally be equipped with end-point detection function, try to avoid over-etching, or more safely, we first deposited an additional layer of silicon nitride or other substances on the chip, used as a stop layer,

Deposition, mentioned here, is the third important process in chip manufacturing, in addition to photolithography and etching, and the three are often interrelated, so I will wait to explain to you more details about dry etching and the more advanced atomic layer etching (ALE) technique until I have first explained the principles of deposition.

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Whether it is a three-dimensional device structure like Gate All Around, or a three-dimensional stack of layers like 3D NAND, the requirements for etching technology will become higher and higher, not only in terms of small size, but also in terms of compactness..

In this issue we will talk about how to dig a hole with etching, in the next issue we will talk about how to fill a hole with deposition, and when we have finished with ion implantation of oxidized metals, we can try to build a simple transistor from scratch.

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