Lithography system

Lithography is a process in the processing of semiconductor chips, and the equipment used is the lithography machine, which is an optical instrument with a complex optical path and sophisticated structure. The resolution of the lithography system is a key performance indicator, determined by factors such as k1, wavelength and numerical aperture (NA).

Lithography is not only one of the most important technologies in chip production, but also the world's most efficient high-precision microfabrication technology, which supports the realization and development of ultra-large-scale integrated circuits. Lithography consists of three important elements: lithography process, photoresist and lithography machine.

Lithography is a complex optical path, well-structured optical instruments, the most common projection exposure system, for example, it is equivalent to a photocopying machine, the mask on the pattern, in a certain proportion of the projection to the photoresist-coated wafers, so as to record the desired pattern in the photoresist layer. A sketch of a complete lithography system is shown below:

In the figure above, you can see that the entire system consists of a laser, beam shaping and attenuation system, optical pupil shaping system, uniformity system, relay mirror, projection objective, wafer and workpiece table and other parts of the laser beam, after a number of shaping, it will be entered into the uniformity system, uniformity system can be turned into a beam in the entire pupil of the beam of energy is very uniform beam. Relay mirror is used to provide uniform telecentric illumination for the mask template, the beam passes through the mask template, and then enters the projection objective, which projects the pattern of the mask template onto the photoresist of the wafer at a scale of 1/4 or 1/5 to complete the exposure. In the entire system, in addition to the commonly used plane steering mirror and spherical lens, will use some special optical components, such as for shaping the cone mirror, W-shaped cone mirror, used to even out the light homogenizer, integrating rod or compound eye lens, etc., it can be said that the entire system uses most kinds of optical components. Lithography system has a very critical performance indicators, is the lithography resolution R (the indicator is also known as the critical size CD, Critical dimension of the abbreviation), the resolution of the formula is as follows:

As can be seen in the above formula, the main three factors that determine the lithographic resolution are k1 ,λ and NA, respectively. k1 is the scale factor, usually determined by the process of lithography, taking the value of the range of 0.25 to 0.5. λ is the wavelength of the light source, the smaller the wavelength, the smaller the resolution. the NA value is the numerical aperture of the projection objective lens, and the larger the NA value of the objective lens, the smaller the resolution of the system. In 1965, one of the founders of Intel Moore had proposed that the number of transistors that can be accommodated on an integrated circuit will double in about every 18 months to 24 months, and the corresponding lithography resolution will be reduced by a factor of two, which is also known as "Moore's Law", and for many years, research and development efforts have been made to improve the level of the lithography process, shorten the lithography wavelength, and reduce the resolution of lithography. Over the years, researchers and developers have worked hard to improve the level of lithography, shorten the wavelength of lithography and improve the numerical aperture of the projection objective to do development, so that Moore's law continues to this day, the path of development of lithography objective is shown in the figure below:

As can be seen from the above figure, the wavelength of the lithography system, from the early g line, i line to the later 248nm, 193nm and 13.5nm EUV light, the wavelength is gradually shortened. The NA value of the projection objective, from the early 0.38, 0.6 to 0.93, with the immersion lithography, NA value increased to 1.35 on the amount, in the EUV band, the recent delivery of NA value of 0.55 high NA lithography system, thus making the lithography resolution from 1000 nm to 100 nm, and then today's 10 nm or less, the integration of transistors on the chip to obtain a The integration of transistors on chips has been greatly improved.