Super-Resolution in Non-Linear Optical Lithography
IC circuits are formed by projecting reduced circuit pattern images. The latest optical lithography instrument can be used to form lines of less than 100nm width with ArF excimer laser illumination.
N.A. = n sin θ Exposure takes place in air, hence the refraction index n=1.
As the width of those lines narrows year by year, the resolution limit according to optics theory is being approached. Among many ways to define the resolution limit, the simplest is 'optical cut-off frequency' : the inverse of the smallest pitch of periodical pattern which can be resolved.
Optical cut-off frequency = 1/P = 2NA/ λ
where P is the pitch of the periodic pattern,
λ is the wavelength of the optical system,
NA is the numerical aperture of the lens.
For camera optics, F-number is usually used instead of NA, where NA=1/2F.
If the pitch of the periodic pattern is smaller than that given in the equation shown above, it is absolutely impossible to obtain the image of the pattern. This means that only two ways remain to raise the resolution limit — shortening λ or increasing NA.
So far, Nikon has proposed a number of well-known resolution enhancement technologies such as phase shifting mask, and SHRINC (quadrupole illumination). Despite the importance of these methods, it is still impossible to exceed the optical cut-off frequency.
Super-resolution technique is the method for raising optical cut-off frequency without changing λ or NA.
There are two approaches to realize super-resolution: near-field exposure using evanescent waves and a method using non-linear response characteristics of the material. The latter is more interesting theoretically.
The principle of super-resolution using non-linear process is simple.
The photo-sensitive material called photo-resist coated on the wafer goes into an exposed state by absorbing a single photon. If we have another type of photo-resist 'whose exposed state' is excited only by absorbing two photons simultaneously, the non-linear exposure process can be observed. The probability for simultaneous arrival of two photons is the square of the probability for arrival of a single photon. Therefore, exposure of this new photo-resist corresponds to the square of the light intensity. This opens the possibility of less dominance of classical optics.
Such an unusual exposure process is the key to super-resolution. To draw an analogy, an optical engineer, like a creative chef, seeks to find the right framework, or cuisine, for this very good material, so that it can be properly applied and appreciated according to need, or taste, in the actual lithography stage.
Our investigation into this intriguing approach is our trump card for prolonging the application of optical lithography.
IC circuits are formed by projecting reduced circuit pattern images. The latest optical lithography instrument can be used to form lines of less than 100nm width with ArF excimer laser illumination.
