Stepper - Illumination and The Challenges of Improving Resolution

Illumination and The Challenges of Improving Resolution

The greatest limitation on the ability to produce increasingly finer lines on the surface of the wafer has been the wavelength of the light used in the exposure system. As the required lines have become narrower and narrower, illumination sources producing light with progressively shorter wavelengths have been put into service in steppers and scanners.

The ability of an exposure system, such as a stepper, to resolve narrow lines is limited by the wavelength of the light used for illumination, the ability of the lens to capture light (or actually orders of diffraction) coming at increasingly wider angles (called numerical aperture or N.A.), and various improvements in the process itself. This is expressed by the following equation:

is the critical dimension, or finest line resolvable, is a coefficient expressing process-related factors, is the wavelength of the light, and is the numerical aperture. Decreasing the wavelength of the light in the illumination system increases the resolving power of the stepper.

Twenty years ago, the ultraviolet "g-line" (436 nm) of the mercury spectrum was used to create lines in the 750 nm range in steppers that employed mercury lamps as their illumination source. Several years later systems employing the "i-line" (365 nm) from mercury lamps were introduced to create lines as low as 350 nm. As the desired line widths approached and eventually became narrower than the wavelength of the light used to create them, a variety of resolution enhancement techniques were developed to make this possible, such as phase shifting reticles and various techniques for manipulating the angles of the exposure light in order to maximize the resolving power of the lens.

Eventually however, the desired line widths became narrower than what was possible using mercury lamps, and near the middle of the last decade, the semiconductor industry moved towards steppers that employed krypton-fluoride (KrF) excimer lasers producing 248 nm light. Such systems are currently being used to produce lines in the 110 nm range. Lines as low as 32 nm are being resolved by production-capable steppers using argon-fluoride (ArF) excimer lasers that emit light with a wavelength of 193 nm. Although fluoride (F2) lasers are available that produce 157 nm light, they are not practical because of their low power and because they quickly degrade the materials used to make the lenses in the stepper.

Since practical light sources with wavelengths narrower than these lasers have not been available, manufacturers have sought to improve resolution by reducing the process coefficient . This is done by further improving techniques for manipulating the light as it passes through the illumination system and the reticle, as well as improving techniques for processing the wafer before and after exposure. Manufacturers have also introduced ever larger and more expensive lenses as a means of increasing the numerical aperture. However, these techniques are approaching their practical limit, and line widths in the 45 nm range appear to be near the best that can be achieved with conventional design.

Ultimately, other sources of illumination will have to be put to use, such as electron beams, x-rays or similar sources of electromagnetic energy with wavelengths much shorter than visible light. However, in order to delay as long as possible the vast expense and difficulty of adopting a whole new type of illumination technology, manufacturers have turned to a technique, previously used in microscopes, for increasing the numerical aperture of the lens by allowing the light to pass through water instead of air. This method, called immersion lithography, is the current cutting edge of practical production technology. It works because numerical aperture is a function of the maximum angle of light that can enter the lens and the refractive index of the medium through which the light passes. When water is employed as the medium, it greatly increases numerical aperture, since it has a refractive index of 1.44 at 193 nm, while air has an index of 1. Current production machines employing this technology are capable of resolving lines in the 32 nm range, and may eventually be able to achieve lines of 30 nm.