Wafer steppers

Equipment Contact /proximity scan projection alignment, wafer stepper... 

Lithographic- Optical wafer steppers are the most expensive and critical machines used in the Fabrication Laboratoiy. 

Firtion, V. A. Jewell, T. E. Wilcomb, B. E. Clemens, J. T. Excimer Laser-Based Lithography A Deep Ultraviolet Wafer Stepper SPIE Conf. on Microlithography, March 13, 1986. 

Pol, High resolution optical lithography a deep ultraviolet laser based wafer stepper, Solid State Technol. 30(1), 71 76 (1987). 

Other characteristics of DNQ/novolac resists that have contributed to their lasting success in the semiconductor industry include their high etch resistance and the fact that they can be developed in environmentally benign aqueous base developers. In addition, the cyclized rubber/bis-azide negative tone resists did not image well at the Hg g-line, the exposure wavelength of the earliest commercially available wafer steppers, the introduction of which effectively brought about the complete and wholesale conversion of the IC industry to novolac resists from the cyclized rubber/bis-azide. ... 

Influenced by the success of the GCA steppers, several companies began to produce wafer steppers and market them commercially. The major stepper suppliers today for advanced lithographic patterning applications include ASM Lithography (ASML), Nikon, and Canon. 

Table 13.10 Comparison of the attributes of the commerciaiiy avaiiabie step-and-scan 193-nm exposure toois and the first commerciai wafer stepper (GCA s DSW4800). it shouid be noted that both Canon and Nikon specify their throughput using 64 and 65 exposure fieids ( shots ) per wafer, respectiveiy, whiie ASML specifies throughput with 125 shots per wafer.

The above problem caused by scattered reflections over existing wafer surface topography is most pronounced with the use of direct wafer steppers using monochromatic exposing radiation. Recently, spun-on antireflective coatings have been shown to be effective in minimizing wafer surface reflections and improving resist performance (31). 

Application IBF technique has been developed for figuring high performance optics components, such as high precision optics used within the optical towers of lithography wafer steppers. Following other mechanical polishing methods, IBF is usually performed as the final step to remove the last surface errors. 

Nowadays, wafer steppers are used for the lithographic structuring of large diameter wafers at high resolution. This equipments use the step and repeat principle. Only small area of the substrate of about 1 cm is exposed to the mask in a single step. Afterwards, the substrate is moved to the next area followed by a subsequent exposure step and so on, which will be repeated until the entire wafer is exposed to the mask. 

In practice, the stepper consists of a machine incorporating a light source, a photomask holder and a lens for focusing the pattern on the photoresist layer on the silicon wafer. The pattern is repeated clear across the wafer, step by step, hence the name. The lens system must be of highest quality so that definition of lines and areas remain accurate and do not overlap each other. 

Optical Exposure. Multicomponent LB films were prepared from solutions of novolac/PAC varying in concentration from 5-50 wt% PAC, and transferred at 2.5 -10 dyn/cm. The films were composed of 15 - 20 monolayers, with an average film thickness of 30 nm, as measured by ellipsometry. Exposures were performed with a Canon FP-141 4 1 stepper (primarily g-line exposure) at an exposure setting of 5.2 and with a fine line test reticle that contains line/space patterns from 20 to 1 pm (40 to 2 pm pitch). They then were then developed in 0.1 - 0.2 M KOH, depending on the PAC content The wafers received a 20 min 120°C post development bake to improve adhesion to the Cr. Finally, the Cr was etched in Cyantek CR-14 chromium etchant, and the resist and Cr images were examined by SEM. 

Lithography In order to precisely resolve the nanometer structures in microelectronics, various enhancement techniques have been applied to the current optical exposure tools that are equipped with deep UV light (193 nm wavelength). These enhancement techniques include phase-shift masks and immersion lenses (putting a liquid between final lens of the stepper and the wafer). The trade-off for the high resolution of modern steppers is an extremely small depth of focus (DOF) that is around 0.5pm over a typical field size of... 

Stepper—A lithographic system that exposes the wafer one small section at a time before stepping to the next location. 

Figure 13.25 How (a) steppers and (b) step-and-scan tools expose a large wafer with a small Image field.

The abbreviation DSW of GCA s first stepper stood for Direct Step on the Wafer, an allusion to the stepper s origin in mask making. 

Infrared spectra were measured using a Nicolet Magna-IR FTIR/550 spectrometer. The resist solutions were spin coated at 2500 RPM to produce -O.Tpm films on 8 inch double polished silicon waiers and heated at 150°C for 60s (unless otherwise stated) in hard contact with the bake plate. Following exposure to known doses ranging from 0-50 mJ/cm on a GCA XLS KrF (248 nm) or an ISI MicroStep (193 nm) exciplex laser stepper, the wafer was baked with minimal delay (<5 min) for 60s at 150 C. This experiment was repeated for post-exposure bake temperatures of 140,130 and 120°C. 

The mask image is transferred to the wafer using a computer-controlled machine known as a stepper. It has a sophisticated lens system to reduce the pattern on the mask (millimetres-centimetres diameter) to the microscopic dimensions of the chips circuitry. The wafer is held in place on a positioning table below the lens system. UV light from an arc lamp or a laser (or X-ray irradiation or electron beam radiation) shines through the clear spaces of the mask s intricate pattern on to the photoresist layer of a single chip. The stepper table then moves the wafer the precise distance required to position another chip under the light. 

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