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A nano-scale sketch on a millimeter-scale wafer Part 2

  • EUV Minimum Pitch Single Patterning by the Samsung Foundry

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Samsung Foundry recently presented a paper on the subject of EUV Minimum Pitch Single Patterning at the International Interconnect Technology Conference (IITC). We’ve prepared this post to help more people understand the paper, as well as the characteristics of EUV technology. 1. If you need to draw a thinner line, bring a thinner brush! In the previous post, we learned about the obstacles faced by the photolithography process. The conclusion was that the fundamental solution to the limitations posed by the nature of light would be shorter wavelengths. This is because wavelength is what determines the degree of diffraction. A shorter wavelength narrows the angle by which light is diffracted, and increases the performance of the photolithography process. You need to use a thinner brush to draw a thinner line. As seen in Figure [1], using a shorter wavelength is the solution to limitations in patterning posed by longer wavelengths.
Figure [1] Using a shorter wavelength in the photolithography process is like using a thinner brush for a sketch.
Figure [1] Using a shorter wavelength in the photolithography process is like using a thinner brush for a sketch.
Figure [1] Using a shorter wavelength in the photolithography process is like using a thinner brush for a sketch.
That’s why advances in the photolithography process have involved reducing the wavelength of the light used in patterning to draw a smaller sketch, that is, a tinier pattern, as seen in Figure [2].
Figure [2] The light source for the photolithography process has evolved from lamps to KrF lasers using Kr (Krypton), then to ArF lasers using Ar (Argon). Each step in the evolution has a shorter wavelength than the previous step.
Figure [2] The light source for the photolithography process has evolved from lamps to KrF lasers using Kr (Krypton), then to ArF lasers using Ar (Argon). Each step in the evolution has a shorter wavelength than the previous step.
Figure [2] The light source for the photolithography process has evolved from lamps to KrF lasers using Kr (Krypton), then to ArF lasers using Ar (Argon). Each step in the evolution has a shorter wavelength than the previous step.
However, even the wavelength of ArF (198nm) was too large to meet the demand for ever-smaller transistors. That’s where EUV (Extreme Ultra Violet) comes in. 2. EUV (Extreme Ultra Violet) to the rescue EUV was the solution we had all been waiting for to achieving shorter wavelengths. The most noteworthy feature of EUV is its short wavelength. Patterning with precision requires a short wavelength, and the reason for adoption of EUV to begin with was to achieve shorter wavelengths. We use EUV that has an exceedingly short wavelength of 13.5nm, as shown in Figure [3].
Figure [3]. Wavelengths relative to the lengths of familiar objects Conventional ArF lasers are classified as DUV (Deep UV), and have a wavelength of 193nm. In contrast, the wavelength of EUV is just 13.5nm - smaller than the size of a molecule.
Figure [3]. Wavelengths relative to the lengths of familiar objects Conventional ArF lasers are classified as DUV (Deep UV), and have a wavelength of 193nm. In contrast, the wavelength of EUV is just 13.5nm - smaller than the size of a molecule.
Figure [3] Wavelengths relative to the lengths of familiar objects Conventional ArF lasers are classified as DUV (Deep UV), and have a wavelength of 193nm. In contrast, the wavelength of EUV is just 13.5nm - smaller than the size of a molecule.
The shift from 193nm ArF to 13.5nm EUV therefore signifies a giant leap. Now let’s take a closer look at the photolithography process made possible by EUV.
A. Short wavelengths created by energetic plasma Figure [3] above shows the rainbow-colored light spectrum we are all familiar with. As we move toward shorter wavelengths, we get ultraviolet rays, responsible for giving us sunburn, then x-rays, which can pass straight through our muscles, and then gamma rays, which are powerful enough to destroy cancer cells. In other words, the shorter the wavelength of light, the stronger its energy. As short-wavelength light has high energy levels, creating light with shorter wavelengths requires larger amounts of energy. It’s similar to baseball. If you want to send the ball farther and faster, then you need to swing the bat harder. But the lasers used to create conventional DUV light had energy levels insufficient for creating the short wavelengths we needed. This is why EUV, as shown in Figure [4], uses plasma (the fourth state of matter, following solids, liquids and gases: in highly energetic plasma, the atoms of matter are separated into electrons and ions).
Figure [4] A CO2 is collided with molten tin (Sn) droplets to generate plasma. The light given off by this plasma is focused using mirrors, giving us EUV light.
Figure [4] A CO2 is collided with molten tin (Sn) droplets to generate plasma. The light given off by this plasma is focused using mirrors, giving us EUV light.
Figure [4] A CO2 is collided with molten tin (Sn) droplets to generate plasma. The light given off by this plasma is focused using mirrors, giving us EUV light.
A special device is needed for the process of creating EUV, as shown in Figure [4]. This special device is none other than a light-focusing mirror. Mirrors are a crucial component not only in creating EUV light but throughout all processes where EUV light is used. Indeed, the mirror is a critical element in EUV technology. In the following, we will take a closer look at the mirror. B. Reflective optical system - Mirrors are used instead of lenses. The shorter the wavelength of light, the easier it is absorbed by other substances. The extremely short wavelength of EUV means it is easily absorbed even in thin air. To keep this from happening, the processes in EUV equipment (photolithography equipment using EUV light) take place in a vacuum. Reducing this absorption of light is also the reason why mirrors are used instead of lenses in EUV processes. EUV has such a short wavelength that much of the light is absorbed by the lens as its passes through. Using mirrors to reflect instead of transmit light reduces the amount of EUV absorbed. Only if absorption is minimized and sufficient EUV light reaches the photoresist can proper patterning occur, as seen in Figure [5].
Figure [5] Up to DUV, lenses were used to focus light. However, the short wavelength of EUV means much of the light is absorbed when passed through a lens. Absorption rates are relatively lower when light is reflected, instead of transmitted. Accordingly, mirrors are used for EUV light.
Figure [5] Up to DUV, lenses were used to focus light. However, the short wavelength of EUV means much of the light is absorbed when passed through a lens. Absorption rates are relatively lower when light is reflected, instead of transmitted. Accordingly, mirrors are used for EUV light.
Figure [5] Up to DUV, lenses were used to focus light. However, the short wavelength of EUV means much of the light is absorbed when passed through a lens. Absorption rates are relatively lower when light is reflected, instead of transmitted. Accordingly, mirrors are used for EUV light.
This raises a question. What about the mask? Isn’t light supposed to pass through the mask? Won’t this cause most of the EUV light to be absorbed? The masks for EUV processes are made to reflect light as well. Figure [6] (a) shows a conventional mask with areas that either transmit or block light. In the EUV process, the mask is made up of areas that either reflect or absorb light.
Figure [6]. To minimize light absorption in an EUV mask, a reflecting mirror made up of multiple layers of Mo (Molybdenum) and Si (Silicon) is used. This mirror is protected by means of a protection layer, which serves as a protective film. In the areas where the EUV needs to be absorbed, an absorber (TaN) is used.
Figure [6]. To minimize light absorption in an EUV mask, a reflecting mirror made up of multiple layers of Mo (Molybdenum) and Si (Silicon) is used. This mirror is protected by means of a protection layer, which serves as a protective film. In the areas where the EUV needs to be absorbed, an absorber (TaN) is used.
Figure [6] To minimize light absorption in an EUV mask, a reflecting mirror made up of multiple layers of Mo (Molybdenum) and Si (Silicon) is used. This mirror is protected by means of a protection layer, which serves as a protective film. In the areas where the EUV needs to be absorbed, an absorber (TaN) is used.
Figure [7] illustrates the EUV photolithography process explained thus far at a glance.
Figure [7] The overall exposure (exposing wafers to light) process in the EUV photolithography process
Figure [7] The overall exposure (exposing wafers to light) process in the EUV photolithography process
Figure [7] The overall exposure (exposing wafers to light) process in the EUV photolithography process
Figure [8] is a brief recap of the differences between EUV and ArF.
Figure [8]. In the ArF photolithography process, light is generated using a laser. The process uses lenses and creates light by a Laser, and it uses a Lens and a transmissive mask. In contrast, EUV light is created using plasma. Mirrors are used, along with a reflective mask.
Figure [8]. In the ArF photolithography process, light is generated using a laser. The process uses lenses and creates light by a Laser, and it uses a Lens and a transmissive mask. In contrast, EUV light is created using plasma. Mirrors are used, along with a reflective mask.
Figure [8] In the ArF photolithography process, light is generated using a laser. The process uses lenses and creates light by a Laser, and it uses a Lens and a transmissive mask. In contrast, EUV light is created using plasma. Mirrors are used, along with a reflective mask.
As shown in Figure [8], the EUV photolithography process is completely different from conventional methods. Using this process, we are able to draw patterns at scales far smaller than before. But allowing smaller patterns than ever before is not the only advantage of EUV.
3. Drawing patterns smaller than ever before, with fewer passes In PART 1 of the post, we discussed a method of using multiple passes to create patterns. This method was devised to overcome the limitations posed by longer wavelengths, and is called MPT (Multiple Patterning Technology). While MPT allows for small patterns to be drawn, it has its drawbacks as well. As shown in Figure [9] (a), the process requires multiple masks, and involves multiple passes (steps) as well. But as shown in Figure [9] (b), the short wavelength of EUV allows for patterns to be created using just a single mask and a single-pass photolithography process.
Figure [9]. ArF requires 4 masks to create a pattern, while EUV is capable of producing the same pattern using just one mask.
Figure [9]. ArF requires 4 masks to create a pattern, while EUV is capable of producing the same pattern using just one mask.
Figure [9] ArF requires 4 masks to create a pattern, while EUV is capable of producing the same pattern using just one mask.
This advantage translates to benefits in terms of time, yield and cost.
A. Time - Reduction of process time requirements Naturally, more steps required to obtain a result will mean a more time-consuming process. A simple metaphor for Figure [9] would be a bakery. In (a), the bakery makes one loaf of bread every four hours. But in (b), the bakery is churning out one loaf per hour. A reduction in steps requires allows for much faster process speeds. B. Yield - Reduced contamination to improve yield Multiple passes means increased likelihood of contamination. It’s like playdough - the more you play with it, the dirtier it becomes. In semiconductor processes, contamination decreases yield. Reducing the number of passes effectively removes multiple yield-impacting factors. C. Cost - Reduced mask production cost Mask production also involves cost. EUVs replace the multiple masks of previous technology with a single mask, and accordingly mask production costs savings are realized too.
4. A good knife must be matched with a good chef. The advent of EUV has brought with it a number of significant advantages. Now, we need to explore and develop means for more effectively harnessing this outstanding tool. In the next post, we will look at the efforts of Samsung Foundry to elevate EUV technology to the next level.
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