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Semiconductor Lithography Systems

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- Overview

A wide range of products such as smartphones and electrical appliances are becoming smaller and more functional, and all are now part of everyday life. Semiconductor lithography technology is making a significant contribution to the evolution of semiconductors (semiconductor integrated circuits), which are indispensable components of these products. A semiconductor lithography system undertakes a process whereby highly complex circuit patterns drawn on a photomask made of a large glass plate are reduced using ultra-high-performance lenses and exposed onto a silicon substrate known as a wafer.

 

- Semiconductor Lithography Systems

Photolithography, also called optical lithography or UV lithography, is a process used in microfabrication to pattern parts on a thin film or the bulk of a substrate (also called a wafer). It uses light to transfer a geometric pattern from a photomask (also called an optical mask) to a photosensitive (that is, light-sensitive) chemical photoresist on the substrate. A series of chemical treatments then either etches the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist. In complex integrated circuits, a CMOS wafer may go through the photolithographic cycle as many as 50 times. 

Photolithography shares some fundamental principles with photography in that the pattern in the photoresist etching is created by exposing it to light, either directly (without using a mask) or with a projected image using a photomask. This procedure is comparable to a high precision version of the method used to make printed circuit boards. Subsequent stages in the process have more in common with etching than with lithographic printing. This method can create extremely small patterns, down to a few tens of nanometers in size. It provides precise control of the shape and size of the objects it creates and can create patterns over an entire surface cost-effectively. Its main disadvantages are that it requires a flat substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions. Photolithography is the standard method of printed circuit board (PCB) and microprocessor fabrication. Directed self-assembly is being evaluated as an alternative to photolithography.


- EUV Lithography Systems

EUV (Extreme Ultraviolet) lithography uses an EUV light of the extremely short wavelength of 13.5 nm. It allows exposure of fine circuit patterns with a half-pitch below 20 nm that cannot be exposed by the conventional optical lithography using an ArF excimer laser.) Putting it into practical use requires a variety of element technologies, including the light source, optics, masks, photoresist, and lithography tools. 

Among these element technologies, the biggest challenge is the technology that generates a powerful EUV beam of the extremely short wavelength of 13.5 nm. This EUV beam can be taken out from high-temperature and high-density plasma. Two methods are used for producing plasma: the Laser-Produced Plasma (LPP) method that produces plasma by condensing a strong laser beam onto a certain material, and the Discharge-Produced Plasma (DPP) method that produces plasma by a pulsed high-current discharge between electrodes in an atmosphere of certain materials. The EUV beam exiting from the plasma is collected by the condensing mirror, passes through a point called the intermediate focus (IF), and illuminates a reflection-type mask after it has been reshaped by the illumination optics. The EUV beam reflected by the mask is exposed by the projection optics to form a pattern on photoresist that is coated on a wafer surface. 

EUV lithography is a soft X-ray technology, which has a wavelength of 13.5nm. Today’s EUV scanners enable resolutions down to 22nm half-pitch. In a system, an EUV light source makes use of a high power laser to create a plasma. This, in turn, helps emit a short wavelength light inside a vacuum chamber. In the chamber, the system uses several multilayer mirrors, which act to reflect light via interlayer interference. 

EUV was originally slated to be introduced at the 45/40nm process node, but problems with the power source to achieve sufficient throughput have forced multiple delays. It was considered a critical component at 16/14nm as a way of avoiding double patterning using 193nm immersion. EUV systems began shipping for 7nm processes. 

At 7nm, single patterning is possible with EUV, but is a relatively slow operation with today’s resists and can cause unwanted random or stochastic defects in patterns, which affect yield.  

At 5nm, double patterning will be required on the critical layers even with EUV. Even though it requires more expensive steps, double patterning means the pitches of features can be relaxed and then processed, which can reduce the number of defects. - 

 
 
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