Photolithography supported by steppers/scanners

Photolithography - the bridge to microfabrication in IC manufacture. The importance of stepper technology cannot be overstated.

The application of photographic technology

In the previous installment of our continuing series on the impact semiconductors have on the IT society, we introduced the major processes involved in IC manufacture. In this article, the spotlight is on photolithography — a key microfabrication process that makes device integration possible. Photolithography, a term created by combining the words "photo" (photograph) and "lithography" (a method of printing using a stone surface), is a type of lithographic printing that employs photographic technology. Simply stated, it is a process of photographic etching performed using light. To explain in a bit more detail, a reticle (or photomask) with the circuit pattern drawn on it in chrome (Cr) is mounted on a silica (glass) substrate, and illuminated. A lens is used to reduce the pattern to a quarter or a fifth of its original size, and the pattern is then transferred to the surface of a resist-coated wafer. The process of photolithography encompasses the preceding steps, as well as the developing step to be explained in the paragraphs to follow (the step in which the exposed photoresist is dissolved). Put another way, photolithography is a technology used to transfer an extremely intricate IC circuit pattern to a wafer and develop it. Once measured in micrometers, the line widths of patterns transferred and developed today have advanced to the nanometer-level (1 nanometer = 10 billionths of a millimeter). The industry is currently in transition from a 65nm to a 45nm process, and is evolving toward 32nm design rules. To give you an idea of just how small IC patterns have become — a distance of 45 nanometers is equivalent to about 1/1,800 the diameter of a single strand of hair.

In production photolithography, anywhere from about a dozen to several dozen reticles are used alternately in the repeated exposure of the patterns onto the wafers, transferring complex electronic circuits (Fig. 1). In order to repeatedly expose detailed patterns with nanometer-level line widths, lenses with extremely high resolving power are required (Fig. 2). Another factor in the ability to create increasingly intricate patterns is the constant reduction in the wavelength of the light source. We have seen an evolution from visible-light g-line to UV i-line, and the wavelengths are becoming even shorter with the advent of laser sources such as krypton fluoride or argon fluoride (Fig. 3). In short, photolithography requires photographic technology, supported by a comprehensive understanding of light and lenses.

The simplest definition of photolithography is "lithographic printing using photographic technology", but when applied to semiconductors it requires some of the most advanced, precise technology ever developed. Countless advances in light sources, lenses, exposure techniques and photoresist have contributed to the evolution of IC chips. Photolithographic innovation: a crucial element in the progress of the Information Technology (IT) society.

Steppers: The driving force behind the continued evolution of photolithography

Let us now turn our attention to the "stepper", an indispensable component in the photolithography process. What, exactly, IS a stepper? The term is derived from "step and repeat," representing the repeated positioning and exposure process performed by the stepper. Here are some visual aids to more clearly illustrate the system and the process itself. First, an overall external view (Photo 1), The transport unit, which contains the wafer and reticle loaders can be seen on the left, and the main unit — including the wafer and reticle stages, light source and projection lens — is in the center, along with the operation control rack. The main unit is enclosed in a sealed chamber that serves to keep dust out, as well as to precisely control temperature and humidity. The control system consists of a programming keyboard, display and other equipment needed to regulate NSR stepper system operation. When the chamber is removed (Fig. 4), it looks like this. The major components are the light source, reticle loader, projection lens, wafer stage and wafer loader. The process begins inside the chamber, where the reticle is first set onto the reticle stage, and then a photoresist-coated wafer is placed on the reticle by the wafer loader. The circuit pattern from the reticle is projected onto the wafer through a lens that reduces it to one quarter or one fifth of its original size. The portions of the resist film exposed to light are then dissolved to reveal the print. This step is known as "developing". It differs from the common developing process, however, in that a positive photoresist is employed, and the portions exposed to light are dissolved. In the standard developing process, the photoresist is negative, and only the portions which aren't exposed to light dissolve. Before the step-and-repeat method became widely used, photolithography was accomplished through contact printing, where the reticle and the wafer were in direct contact with each other, or by proximity printing. Both of these earlier types of photolithography began from full-scale (1:1) exposure. Step-and-repeat technology employing a reducing projection lens was developed to increase the integration level of the ICs being manufactured, and the machines that performed the process became known as steppers. Scan (step-and-scan) systems were later developed to provide even higher resolution on broad exposure areas. Today, both step-and-repeat and step-and-scan systems may be used, depending on the required resolution (Fig. 5).

A stepper must provide high performance in the following three areas: 1. High-resolution projection optics, to transfer intricate electronic circuit patterns. 2. Highly accurate alignment, so that each pattern is precisely aligned with the ones below it. 3. High-throughput capability, for optimum efficiency in mass production. We've barely scratched the surface where optics are concerned, so let's take a closer look. 1. High-resolution projection optics, to transfer intricate electronic circuit patterns. The degree of intricacy of a circuit pattern that can be transferred is indicated by resolution, which is affected by the wavelength of the light source used for exposure. The shorter the wavelength, the higher the resolution. In a stepper, the performance of the projection lens has a significant impact on resolution, making optical design crucial. The most advanced projection lenses designed for use with the highest resolution measure 200 millimeters or more in diameter, consist of 20 or more individual lenses, and are almost a meter in length. Nikon handles every stage of manufacture for these lenses, from the mixing of raw materials to melting, polishing, coating and assembly. 2. Highly accurate alignment, so that each pattern is precisely aligned with the ones below it. In photolithography, the electronic circuit patterns are transferred to the wafer via repeated exposure, and high-precision alignment capability (at the nanometer level) is essential in ensuring correct positioning. Laser light and image processing sensors are used to measure the alignment position of the marks on the wafer, and make any necessary adjustments. The NSR-S308F, for example, boasts alignment precision of 8nm or better. Enhanced lens resolution means shallower focus depth, but even at the highest resolution the more intricate patterns cannot be properly exposed without tight focus. However, it isn't practical to move the projection lens as it's rather large and heavy, so the stepper is designed for the wafer to be moved to ensure accurate alignment (Fig. 6). 3. High-throughput capability, for optimum efficiency in mass production. "Throughput" expresses the processing capacity of a stepper. The most advanced steppers are extremely expensive — in the neighborhood of ¥2 billion per system — and to deliver the performance customers demand at that price the steppers must be designed with productivity high on the list of priorities. In the case of a stepper, throughput is shown most commonly in this form — ex. 120 wafers/h. — the number of wafers it can produce in an hour's time. As noted earlier, each wafer undergoes repeated exposure. As such, throughput depends upon three key factors: 1) the speed of the stage being stepped; 2) the duration of each exposure, and 3) the number of times the wafer must be exposed. It is difficult to reduce the number of exposures as the area of the wafer to be exposed is fixed, so other measures must be taken to increase throughput. Two such examples are boosting the stage speed, and reducing the time required for exposure by employing brighter illumination sources.

Next-generation ICs and steppers

The world's first microprocessor, created in 1971, consisted of 2,300 transistors. By the year 2000, the number of transistors per chip had grown to between 42 million and 55 million, and in 2005, the figure surpassed 600 million. As the integration level of IC chips continues to rise, steppers will continue to play a significant role. High resolution for finer patterns The minimum line width — or maximum resolution — of electronic circuit patterns has reached 90nm. The target for the year 2010 has been set at 25nm, the distance 65 silicon atoms would occupy if arranged in a line. Stepper technology is on the verge of entering the atomic domain (Table 1). In search of light sources with shorter wavelengths Light sources used to this point include ultraviolet and laser (excimer lasers like krypton fluoride and argon fluoride). However, future evolution will require sources with even shorter wavelengths, such as extreme ultraviolet (EUV) (Table 2). The wavelength of i-line light is 365 nanometers, but the wavelength of EUV is markedly shorter and will make possible the exposure of even more intricate patterns.

Immersion, the next generation in photolithography One index of performance for lenses used in steppers is the numerical aperture, or NA. The higher the NA, the higher the resolution. In air, a maximum NA of about 0.9 is generally believed to represent the physical limit. Pure water, however, has a much higher refractive index than air, and placing such water between the lens and the wafer improves the NA to 1.0 or over, resulting in extremely high resolution. This technique is referred to as immersion lithography (Fig. 7). Steppers have supported the continuing evolution of integrated circuits, and through them, the evolution of modern society itself. The very essence of Nikon lens technology, along with precision machining and measurement technologies, is built into each stepper. It's not hard to understand why the stepper is widely considered to be the "most precise machine ever created". Nikon first introduced the NSR-1010G in 1980. Since then, Nikon has developed steppers in accordance with continuing advancements in IC integration. In 2005, the ArF Immersion Scanner NSR-S609B was marketed for production of the latest devices.