Step-and repeat systems

Figure 7. Schematic of a reduction step and-repeat system.

If distortions of the sample are truly isotropic, the only errors that remain after magnification is corrected are residual distortions in the optics, and mask errors. The easiest way to reduce mask errors is to go to 5X or 10X step and repeat systems, but eventually, it should be possible with electron beams to reduce mask errors to insignificant levels. Distortion in the optics of the model 500 is already below 0.1 pm and 3[Pg.15]

Alternatives for the next generation of lithographic systems include 248-nm radiation, 5 X -reduction step-and-repeat systems, and electron beam (e-beam) or X-ray technologies. The 248-nm deep-UV option is most likely to become the next major technique. e-Beam and X-ray technologies will also be important in future lithographic strategies however, when and to what extent they will be commercially significant is not well defined as yet. 

In Figure 1.4, some examples of imprinted resist layers, made by our group on an EVG 770 step-and-repeat system, are illustrated. With an optimized initial resist thickness and a known mold design, an excellent contrast between the feature height and the residual layer thickness can be obtained this is a very important point in the case of additional process steps. 

Figure 3. Schematic of present and potential future optical lithography systems (a) Perkin Elmer Micralign (10), (b) Bell Labs printer (11), (c) reduction step-and-repeat (Censor, Electromask, GCA, Optimetrix, Philips), (d) IX step-and-repeat (Ultratech), (e) IX stripe scan, and (f) reduction step-scan, R indicates object and image orientations. Lenses are indicated only schematically. (Reproduced with permission from Ref. 30)...

Figure 40. Operating modes for electron beam systems left — raster scan coupled with continuous table motion right — vector scan, step and repeat.

Several other successful machines have also been reported using raster scanning. The IBM EL-1 is a step-and-repeat type system that exposes a 5 mm x 5 mm field. These small fields are then stitched together to form the complete circuit. The EL-1 systems use a 2.5 /im x 2.5 fim square beam and are used to manufacture custom circuits. 

Deep-UV source brightness is another issue, because the power output of a 1-kW mercury-xenon lamp in the 200-250-nm range is only 30-40 mW. For this reason, excimer lasers (such as KrCl and KrF), which can deliver several watts of power at the required wavelengths, are being considered as alternatives (7). In fact, a deep-UV step-and-repeat projection system with an all-quartz lens and a KrF excimer laser with an output at 248 nm has been reported (8). Even the laser-based systems require resists with a sensitivity of 30-70 mj/cm2. 

Pol et al. 10) developed an excimer laser-based DUV projection system by modifying a commercial step-and-repeat exposure tool (GCA model 4800 DSW). This system is equipped with a KrF (248 nm) excimer laser, an all-... 

The year 1972 witnessed the invention of proximity x-ray lithography by Hank Smith at Massachusetts Institute of Technology. Many companies (IBM, Canon, Nikon), universities (MIT and the University of Wisconsin), and governments (United States and Japan) spent well over a billion dollars on its development. The first attempts at production of x-ray aligners were made in the mid-1970s by AT T and MIT, and in the early 1980s by Micronix Partners, Hampshire Instruments, Nikon, and Karl Suss. X-ray step-and-repeat and step-and-scan systems were eventually made commercially available. ... 

KrF exposure systems (step and repeat) already exist commercially and are used for 0.25 pm linewidth generation. But most emphasis has been recently put on the 195 nm exposnre systems for 0.18 pm linewidth generation. The next generation of components will reqnire linewidths below 0.12 pm. 

A step-and-repeat process allows the fabrication of smaller and cheaper stamps and a better control of the placement accuracy [8]. Such a process is easier to implement with a UV system than with a thermal system. 

Planarization of the device topography before lithographic imaging has become an extremely important technique as the devices have become more uneven and the linewidths have become smaller. A resist image which crosses (lies on top of) a topographic feature on the device will be distorted after development due to the variations in resist thickness because of the unevenness of the surface on which the resist was coated. For a positive resist this means that the thin resist areas, such as the top corner of a step, will have smaller critical dimension lines than the thick resist areas at the bottom of the step (Figure 1In addition, step and repeat exposure systems, which are the most sophisticated systems for optical exposures, have a very limited depth of focus and only function optimally on flat resist coatings. 

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