More Productivity at Subcritical Layers

Suppliers are finding ways to reduce COO for subcritical layers.

Ruth DeJule Associate Editor -- Semiconductor International, 4/1/1998

While the industry focuses on advanced technologies for critical layers, responding to shrinking device geometries, there is still room to improve productivity in subcritical layer processing. This arena presents a challenge to photoresist and lithography tool manufacturers who are continually developing methods to improve process control and reduce cost-of-ownership (COO).

Exposure tools for subcritical layers are being reconfigured to enhance productivity. Lithography tool manufacturers such as Nikon (Belmont, Calif.) and ASML (Tempe, Ariz.) have developed i-line step-and-scan systems to be used in a mix-and-match scheme with DUV scanners. ASML, for example, recently introduced the PAS 5500/400, a high-resolution product with machine-to-machine overlay of <70 nm, resolution as low as 0.28 Å and throughput of 96 wph. Analysis of a 20-layer, 0.22 Å DRAM device demonstrated that the scan system could image 13 of the device's noncritical layers at resolutions as small as 0.28 Å. The company's DUV step-and-scan imaged the remaining seven critical layers. Operating costs with the i-line product averaged ~$5/layer vs. $7.50 for DUV. Thus, resolving as many layers as possible with an i-line tool is advantageous, resulting in operating cost reductions of ~15% over a conventional DUV approach.

1. Lamp DUV, an alternative to advanced i-line systems, can have a broader process latitude.

Another approach is using lamp DUV. Silicon Valley Group (SVG, San Jose, Calif.) has modified a Micrascan DUV system competitively priced in the category with new advanced i-line tools. By using a unique catadioptric lens in conjunction with a refractive system, a filtered mercury xenon lamp can be used for designs with numerical apertures as high as 0.5 and can accommodate chromatic correction, thus enabling the lamp to be used at full power capacity, noted John Shamaly, vice president of marketing at SVG. For subcritical layer processing, this DUV lamp-based system eliminates the expense and productivity restrictions associated with using multiple resist chemicals for i-line and DUV processing. In addition, DUV processing, whether lamp- or laser-based, will always provide better resolution, as defined by the k₁ factor (Fig. 1). Finally, a considerable COO savings may be realized in terms of consumable materials. SVG estimates that cost in consumables for a lamp-based system runs ~$60,000/year, compared to $300,000 for a laser-based tool.

Similarly, photoresists are being fine-tuned for greater process latitude. Specialty photoresists are designed to give good performance for a single exposure wavelength and development process, generally optimized at g-, h- or i-line. Crossover resists, capable of being used at more than one wavelength, have been commercially available, but with limited process latitude. Today, more robust multiwavelength resists have been developed that provide a wider process window for exposures from broadband to i-line. Use of a crossover photoresist can facilitate consolidation of multiple chemical systems and reduce material costs 24% or more, noted Kathryn Yager, product manager at AZ Electronic Materials, a business unit of Clariant Corp. (Somerville, N.J.).

For the past few years, photoresist manufacturers such as AZ Electronic Materials have characterized different types of novolak resins and photoactive compounds. Advances in technology have meant greater sophistication in analysis and characterization of these raw materials, which has allowed the engineering of new material combinations for use at multiple exposure wavelengths and the selection of resins to meet specific targets of photospeed and thermal stability.

Intended for use in subcritical layer applications, >0.5 Å, the AZ 3312 resist is suitable for a wide range of exposure conditions (Table 1) and both wet and dry etch processes. Thermal stability up to 125Å also enables this resist to be used for most ion implant processes. This product is designed to work with TMAH-based developers, with and without surfactants, or inorganic developers. Harris Semiconductor (Findlay, Ohio) is implementing the AZ 3312 resist in a broad range of applications. Preliminary data indicate that this crossover resist system is "excellent for broadband exposure applications in both lithographic performance and cost," said Jim Reed, lead process engineer at Harris, "and looks to be a strong performer in both g- and i-line applications."

Table 1. Crossover Resist: Performance Per Exposure Tool