How did lithography become the card-neck technology of chip manufacturing step by step?

Abstract: There are many technologies used in chip manufacturing. Lithography is the soul technology of chip manufacturing, but at the beginning, lithography is not the most powerful of all technologies. Now that the public recognizes the importance of chips, when discussing the problem of card necks in the chip industry, lithography and lithography machines are the most mentioned. So, how did lithography become the card-neck technology of chip manufacturing step by step? This article attempts to find out.

Photolithography technology uses the principles of optics and chemical reactions, as well as chemical and physical etching methods, to fabricate circuit patterns on the semiconductor substrate or the next layer of dielectric materials, and superimpose them through multiple photolithography processes in an orderly manner. Finally, the three-dimensional circuit structure is fabricated on the semiconductor substrate to form a complete functional chip.

How did lithography become the card-neck technology of chip manufacturing step by step?

 Figure 1. Schematic diagram of the photolithography process

Figure 1 is a schematic diagram of the photolithography process. Assuming that a T-shaped pattern of silicon dioxide (SiO2) is to be etched on a silicon (Si) substrate, roughly 6 processes are required. Before photolithography, a layer of SiO2 is first grown on the Si substrate, and then starting from the photoresist coating, go through the steps of coating, exposing, developing, removing the gel, etching, and removing the gel. As can be seen from the figure, the fourth step and the previous process delineate the area to be removed for the next step of etching the SiO2 layer. The fifth step is the actual processing process-etching, the sixth step is the finishing process, the purpose is to remove the unnecessary photoresist on the SiO2 T-shaped pattern. Therefore, lithography has two stages, one is to define the scope, and the other is to perform actual processing for the defined scope.

The above-mentioned photolithography process is very similar to the exposure and phase washing process of traditional photography, and it is not difficult to understand. But photography is a transitional optical and chemical reaction process, and the more transitional levels, the more delicate the photo and the better. Lithography is an abrupt optical and chemical reaction process, requiring the mutation as fast as possible and the clearer the pattern boundary, the better. In order to pursue the goal of clearer borders and finer lines, lithography technology has gone through a long road of technological innovation.

How did lithography become the card-neck technology of chip manufacturing step by step?

  Figure 2. Multiple photolithography processes are “stacked” to form a three-dimensional circuit structure

2 is a schematic diagram of a three-dimensional circuit structure formed by “stacking” multiple photolithography processes. Figure 2 (a) is a layout of a transistor. The layout is divided into at least 5 layers of masks according to the manufacturing process, as shown in Figure 2(b). In the chip manufacturing process, a photolithography process is arranged for each layer of mask. After five photolithography processes, three-dimensional transistors are “stacked”, as shown in Figure 2(c).

In layman’s terms, chip manufacturing is to use photolithography to “engrave” a layer of material to form specific patterns on a semiconductor substrate. Layer by layer photolithography actually “stacks” these patterns vertically to form three-dimensional transistors and circuits. components and connections, etc., eventually form a chip with complete circuit functions.

  1. Why is lithography technology the soul technology?

Lithography technology is the soul technology in chip manufacturing. Without its existence, chip technology would not exist and develop rapidly. The reason why lithography is the soul technology is that lithography needs to define the processing range for other chip processing technologies, and lithography is as important as the aiming device of a gun. No matter how complicated and difficult other processing technologies are, they can only play a role under the premise of the existence of photolithography. For example, it is necessary to rely on photolithography to determine the pattern, position and orientation of the poly gate and metal connection (Metal) of the transistor; to rely on photolithography for the diffusion area (Diffusion), implant well (Implant Well), upper and lower layers Via hole (Via, Contact) opens the processing window and so on. Therefore, there is no other processing technology without photolithography technology.

From the transmission of chip design data to chip manufacturing process, the transmission path is very clear, that is, chip design layout -> mask -> lithography -> processing. The design layout of a chip must be decomposed into a set of multiple (layer) masks according to the manufacturing process, and each mask corresponds to a photolithography and processing process. Therefore, lithography is the soul technology of chip manufacturing.

In the 21st century, with the development of semiconductor technology, the accuracy of lithography has been continuously improved, and has been refined from the micron, submicron, and deep submicron levels to the current nanometer level. The light source used for lithography has also changed from the conventional The light source has developed to the application of new technologies such as electron beams, X-rays, micro-ion beams, and lasers. Lithography has become the most precise microfabrication technology and the most critical technology for chip manufacturing. If the core equipment and materials of lithography are monopolized and controlled by individual countries, lithography technology will become the key core technology that “cards” the “neck” of the chip industry in other countries.

 2. What technologies are still used in chip manufacturing?

In addition to photolithography technology used in chip manufacturing, there are many other technologies, such as etching, oxidation, diffusion, deposition, ion implantation, and so on. 1. Etching is the process of selectively removing unwanted materials from the surface of semiconductor materials by chemical or physical methods. Combining photolithography and etching, the pattern on the reticle can be copied correctly on the semiconductor material; 2. Oxidation is the formation of an oxide film in the designated area; 3. Diffusion is the quantitative doping of other element atoms in the designated area to change the electrical properties of the area; 4. Deposition is the deposition of a layer of silicon oxide, silicon carbide, and polysilicon in the designated area And other thin layers of semiconductor materials; 5. Ion implantation is to quantitatively inject impurity atoms or particles into a designated area to change the electrical properties of the area. Compared with diffusion, ion implantation has no spillover effect, and the depth and amount of ion implantation are better controlled.

In the early days of chip technology development, lithography was not the most powerful of all technologies. Restricted by the primitive and simple mask production and manufacturing process, the chip process adopts millimeter and micrometer level, so the number of integrated circuit elements such as transistors on the chip is also extremely limited. Therefore, it is feasible to use simple methods such as manual (or computer-assisted) red film engraving, microphotographic plate making, and traditional light source exposure in lithography. However, the technical issues related to equipment, materials, and control of etching, oxidation, diffusion, deposition, ion implantation and other technologies are more difficult to grasp. At that time, these technologies became the most concerned and innovative directions in chip manufacturing.

 3. What development stages have chip technology gone through?

Chip manufacturing technology has followed the lithography technology all the way, and roughly experienced the very primitive “stone age”, through the computer-assisted semi-automated era, and then entered the EDA fully automated era. my country’s chip technology started roughly at the same time as abroad. Due to well-known reasons, our development stage is basically behind that of foreign countries for about 10 years. Since the reform and opening up, we have accelerated the pace of learning and catching up. At present, the application level of my country’s lithography technology has basically kept pace with foreign countries, but the core technology and equipment basically depend on foreign countries.

  1. The “Stone Age” of chips (from 1958 to 1975 abroad, and from 1958 to 1985 in China). It is called the “stone age” of the chip industry because the chip design tools and the method of making photolithography masks are very primitive. During this period, ordinary graph paper was used for chip design, as shown in Figure 3(a), the layout was drawn by hand; Rubylith was used when making the mask, as shown in Figure 3(b). The red film is an extremely thin red film attached to the transparent substrate. The red film has strong light-shielding. If the boundary of a certain pattern of the red film is cut (without cutting the substrate), the red film outside the pattern can be peeled off from the transparent substrate. The area outside the figure becomes transparent, and the area left by the red film is used as a shading part, which can be made into a transition mask. It undergoes microphotographic plate making and is finally made into a working mask.

    How did lithography become the card-neck technology of chip manufacturing step by step?

  Figure 3. Early use of graph paper to draw the chip layout, manually engrave and check the transition mask (red film)

At that time, when people were designing chips, they first had to draw the chip layout on graph paper. Different layers on the layout were represented by different lines or hatchings. This process is called designing the layout or drawing the layout. Then, copy the layout on the graph paper to different sheets of red film in a 1:1 ratio, and then manually engrave the unnecessary parts on the red film, leaving only the graphics on the layout. This process is called engraving. Red film. The multilayer red film constitutes a set of transition masks for the chip.

Due to the large size of the transition mask, many times of miniature photography are required, and finally the graphics on the transition mask are reduced and copied to the working mask. The working mask is made of a very flat quartz glass plate with a layer of chromium film. The graphic on the red film is exactly the same as the graphic on the chrome film, with different proportions. The area with chrome film does not transmit light, and the area without chrome film is transparent. The working mask is the mask used in the photolithography process when manufacturing the chip.

In the “Stone Age” of chip manufacturing, chip layouts were drawn manually, and patterns on the red film were manually carved. It was impossible to design and manufacture large-scale chips. The number of transistors integrated on this small-scale (SSI) chip Approximately between tens to hundreds. In 1971, Intel introduced the world’s first 4-bit central processing unit (CPU) chip 4004. It integrates 2250 transistors and adopts a 10μm process, which was already a very large-scale chip at that time. The 4004 chip internal photo is shown in Figure 4. The author is not sure whether the mask of the chip is made by artificially engraving the red film. If the red film is engraved manually, then its workload is very huge, but it is also feasible. Think about the situation when people in the room were planning to calculate atomic bomb data when my country was developing two single satellites. This is nothing!

How did lithography become the card-neck technology of chip manufacturing step by step?

 Figure 4. Internal photo of the Intel4004 processor chip (Source: sisite.tw)

2. Computer-aided era (around 1975-1990 abroad, around 1985-1995 in China): In this period, when people design chips, they can either use graph paper to draw the layout, or design the layout on the computer, and the layout on the graph paper. To input into the computer through a digitizer, the layout in the computer can be checked or modified on the computer screen (such as L-EDIT software, etc.), or the simulation of circuit devices such as transistors (such as SPICE software, etc.). Finally, the computer controls the plotter to draw the layout for inspection or archiving, or the red film is engraved by the engraving machine, and finally through the microphotograph, the transition mask is converted into a working mask.

Figure 5 (a) is the user interface of Tanner Research’s layout editing software (L-EDIT). Different mask layers in the layout are represented by different colors or filling patterns. Figure 5(b) is a scene where the designer uses L-EDIT to draw and check the chip layout on the computer.

How did lithography become the card-neck technology of chip manufacturing step by step?

 Figure 5. Use L-EDIT software to design the layout on the computer

In 1974, Intel introduced the 8-bit CPU chip 8080, which was manufactured using a 6μm process and integrated more than 6000 transistors. Figure 6 (a) is an internal photo of the 8080 processor chip. Figure 6(b) is a group photo of the three founders of Intel and a mask (red film) of 8080CPU. Figure 6 (c) is the package appearance of the 8080 CPU.

How did lithography become the card-neck technology of chip manufacturing step by step?

 Figure 6. The three Intel founders pose for a group photo behind a mask (red film) of 8080CPU (from left to right are Andy Grove, Robert Noyce, Gordon Moore, source: sisite .tw)

3. The era of design automation (from 1990 to today in foreign countries, from 1995 to today in China): During this period, people generally used Electronic design automation (EDA) software to complete chip logic design, simulation and layout design on computers, and design data Send to the manufacturing plant, complete the production of the chip mask through automated equipment, and finally the entire set of masks will be used in each photolithography process in the chip manufacturing.

During this period, through continuous integration, mergers and reorganizations, many EDA companies have been scouring the sand, forming the current three giants of foreign EDA software vendors. Synopsys, Cadence and Mentor Graphics have emerged as three dominant EDA software markets in the world. They entered the Chinese market from 1990 to 1995 and accelerated the development of China’s chip design industry. During this period, the chip manufacturing process was rapidly iterated and upgraded along Moore’s Law, and the characteristic line width of the process was reduced from 0.35μm to today’s 5nm. The two key supporting conditions for these developments are EDA software and photolithography machines.

How did lithography become the card-neck technology of chip manufacturing step by step?

  Figure 7. Lithography masks used in automated production lines

How did lithography become the card-neck technology of chip manufacturing step by step?

 Figure 8. The legend of the whole process EDA software design chip

The current chip design process, from the functional design of the chip, to the circuit structure design, to the physical realization of the chip layout, is all done with the help of EDA software, which also includes complex and accurate design checks, simulations, etc., which can guarantee the accommodation A chip design with tens of billions of transistors is foolproof.

In modern lithography technology, mask production has realized the automation of data output by EDA software -> mask production, and the lithography process has realized automation by lithography machines and etching machines. At present, ASML EUV lithography machine has realized the 7nm, 5nm process, and the new generation EUV lithography machine under development can realize the 1nm process. Therefore, today’s chip technology is in an era of fully automated design and manufacturing.

 Fourth, Moore’s Law makes lithography the king

In the three development stages of chip technology, the principle of lithography technology is simple and clear, and has always remained the same. What has changed is that the lithography lines are thinner and the lithography accuracy is higher. People’s demand for smaller, faster and better chips is the only driving force for the innovation and development of lithography technology, and the law of development is Moore’s Law.

Moore’s Law predicts that chip integration will double in less than every two years, and the characteristic line width of the manufacturing process will be halved or reduced under the condition that the chip area remains unchanged. It is inevitable that the lithography accuracy must be doubled in less than two years, and strict requirements are put forward on the lithography technology and equipment.

 Figure 9. Map of chip manufacturing process nodes (Source: m.sohu.com)

From Figure 9 chip manufacturing process node map, you can roughly see the shadow of lithography machine and lithography technology progressing year by year, and you can also feel that it is Moore’s Law that pushes lithography technology step by step to an increasingly important position. After entering the 14nm process node, the technical difficulty of the lithography machine has risen sharply. The price of the ASML EUV lithography machine has reached 120 million US dollars. The equipment cost of the lithography machine accounts for 35% of the cost of all manufacturing equipment, and the lithography process accounts for all manufacturing man-hours. About 40% of it. Lithography technology has become the true king of chip manufacturing, and therefore has become the technology that is most likely to be “stucked” in chip manufacturing.

Conclusion: The chip industry has been following the laws of Moore’s Law. After the characteristic line width of the manufacturing process crosses the current nodes of 7nm, 5nm, and 3nm, lithography technology will face the challenge of physical limits, and lithography technology has become a part of the chip industry. The technology king. Since lithography technology and equipment are mainly monopolized by foreign companies, lithography has become the card neck technology of my country’s chip industry. How to solve this stuck neck problem requires strong support from the state and the hard work of scientific and technological personnel. At the same time, it also indispensably requires a long time of technology accumulation.

  

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