Roderick R. Kunz

Lithography with 157 nm lasers

Projection photolithography at 157 nm was studied as a possible extension of current 248-nm and planned 193-nm technologies. At 157 nm, lasers are available with ∼8 W average power. Their line width is narrow enough as to enable the use of catadioptric, and maybe all-refractive optics similar to those used at 248 and 193 nm. The practicality of such designs is further enhanced by measurements of calcium fluoride, which show that its absorption is sufficiently small (∼0.004 cm−1) at 157 nm. Binary masks with chromium and chromeless phase shifting masks were fabricated on calcium fluoride as the transparent substrate. Robust photoresists at 157 nm still need to be developed, and they probably will be of the top surface imaging or bilayer type. Indeed, a silylation resist process was shown to have characteristics at 157 nm similar to those at 193 nm. The calcium fluoride based masks were integrated with the silylation process and a home-built, small-field, 0.5-numerical aperture stepper to provide projection...

Large-area interdigitated array microelectrodes for electrochemical sensing

Abstract Advanced photolithography developed for the semiconductor industry has been used to fabricate interdigitated microelectrode arrays that pass steady-state limiting currents of up to 230 nA/μM analyte — 2.5 times more than the most sensitive interdigitated array built to date, and exhibit response times of ∼5 ms. This performance results from the small interelectrode gap and the large active area of the device (4 mm2), a combination enabled by advanced photolithography. We describe the fabrication of these arrays and the characterization of their performance in two environments: an aqueous solution of Ru(NH3)63+ and a dinitrotoluene solution in acetonitrile. The scaling of array performance parameters with device dimensions is also presented.

Critical issues in 157 nm lithography

Projection lithography at 157 nm is a candidate technology for the 100–70 nm generations, and possibly beyond. It would provide an evolutionary extension to the current primary photolithographic processes and components: excimer lasers, refractive optics, and transmissive masks. This article presents data on the transmission of optical materials at 157 nm, the performance of optical coatings, the issues that must be faced by photomasks, and the considerations related to engineering resists at this wavelength.

Outlook for 157-nm resist design

We have measured the transparencies of a number of a candidate resist materials for 157 nm, with an emphasis on determining which chemical platforms would allow resist to be used at maximum thicknesses while meeting requirements for optical density. Assuming an ideal resist optical density of 0.4, our findings show that all existing commercially available resists would need to be < 90 nm thick, whereas specialized hydrocarbon resists could be made approximately 120 nm thick, and new resists based on hydrofluorocarbons, siloxanes, and/or silsesquioxanes could be engineered to be used in thicknesses approaching 200 nm. We also assess the tradeoff between these thicknesses and what current information exists regarding defects as a function of resist thickness.

Line-edge roughness in sub-0.18-μm resist patterns

We have characterized line-edge roughness in single-layer, top-surface imaging, bilayer and trilayer resist schemes. The results indicate that in dry developed resists there is inherent line-edge roughness which results from the etch mask, resist (planarizing layer) erosion, and their dependence on plasma etch conditions. In top surface imaging the abruptness of the etch mask, i.e., the silylation contrast, and the silicon content in the silylated areas are the most significant contributors to line-edge roughness. Nevertheless, even in the case of a trilayer, where the SiO2 layer represents the near ideal mask, there is still resist sidewall roughness of the planarizing layer observed which is plasma induced and polymer dependent. The mechanism and magnitude of line-edge roughness are different for different resist schemes, and require specific optimization. Plasma etching of silicon, like O2 dry development, contributes to the final line-edge roughness of patterned features.

Electrical, crystallographic, and optical properties of ArF laser modified diamond surfaces

Pulses of 193 nm radiation from an ArF laser with energies exceeding 0.5 J/cm2 have been shown to modify 40–60 nm thick layers of {100} and {110} oriented diamond surfaces. These layers exhibit highly anisotropic electrical and optical properties which have principal in‐plane axes along the 〈110〉 directions. The minimum resistance is (4–10)×10−4 Ω cm, and minimum in the optical transmittance and maximum in the reflectance occur when the electric field vector of the incident polarized light is aligned along the low resistance direction. Transmission electron microscopy indicates that the modified layer primarily consists of unidentified graphite‐like carbon phases embedded in diamond. The first‐order electron diffraction spots correspond to lattice spacings of 0.123, 0.305, and 0.334 nm. The modified layer is stable at 1800 °C, forms ohmic contacts to type IIb diamond, and supports epitaxial diamond growth.

Immersion liquids for lithography in the deep ultraviolet

The requirements of liquids for use in immersion lithography are discussed. We present simple calculations of the transmission and index homogeneity requirements of the immersion liquid (T > 0.95 and δn < 5×10-7 respectively for sin θ = NA/n = 0.9 and a working distance of 1 mm) along with the temperature and pressure control requirements which follow from them. Water is the leading candidate immersion liquid for use at 193 nm, and we present data on its chemical compatibility with existing 193 nm resists through dissolution/swelling and surface energy studies. We find that it has a minimal impact on at least some current 193 nm resists. At 157 nm, suitably transparent immersion fluids remain to be identified. Perfluorinated polyethers (PFPE) are among the most transparent organics measured. The lowest PFPE absorbance at 157 nm can be further reduced by roughly a factor of two, from 6 to 3 cm-1 through removal of dis-solved oxygen. We also discuss our efforts to understand the origin of the remaining absorbance through supercritical CO2 fractionation.

Extending optics to 50 nm and beyond with immersion lithography

Numerical imaging simulations demonstrate the capability of immersion lithography to print features smaller than 45 nm (35 nm) with good depth of focus at a vacuum wavelength of 193 nm (157 nm). The optical impact of index variation of the immersion liquid is simulated and found to be a shift of focus of 1 nm for each 1 ppm change in the bulk index of the liquid. For an index which varies through the thickness of the liquid (e.g., due to nonuniform temperature), the focus shift is found to be proportional to the total change in optical path length (OPL), with a 1 nm change in OPL leading to a ∼1.5 nm focus shift at 1.3 numerical aperture. A focus offset of 1–3 nm can be expected due to heating during scanning exposure. The possible formation of nanobubbles at resist surfaces is also discussed. While simulations show that even 10 nm thick bubbles at the surface of the resist cause 30% modulation in the aerial image intensity, no evidence of bubbles is seen in open frame immersion exposures. Imaging of 100 ...

Photoresists for 193-nm lithography

Photolithography using 193-nm light appears to be a viable route for the extension of optical lithography to the dimensions required for the manufacture of 1Gb DRAM and advanced CMOS microprocessors with 180-140-nm minimum feature sizes. In this paper, we discuss the origin of resist technology for 193-nm lithography and the current status of 193-nm photoresists, focusing on single-layer resist materials. We emphasize the photoresist design approaches under investigation, compare these with deep-UV (DUV) (248-nm) resist design and materials, and consider possible future lithography processes employing 193-nm lithography. Research and development on 193-nm photoresists by the lithography group at the IBM Almaden Research Center is highlighted.

Limits to etch resistance for 193-nm single-layer resists

An important aspect of single-layer resist use at 193-nm is the inherently poor etch resistance of the polymers currently under evaluation for use. In order to provide the information necessary for resist process selection at 193 nm, we have projected the ultimate etch resistance possible in 193-nm transparent polymers based on a model we have developed. First, a data base of etch rates was assembled for various alicyclic methacrylates. This data base has been used to develop an empirical structure-property relationship for predicting polymer etch rates relative to novolac-based resist. This relationship takes the functional form normalized rate equals -3.80r3 plus 6.71r2 minus 4.42r plus 2.10, where r is the mass fraction of polymer existing as cyclic carbon. From this analysis, it appears as though methacrylate resists equal in etch resistance to deep UV resists will be possible. Early generations of methacrylate-based 193-nm resists were also evaluated in actual IC process steps, and those results are presented with a brief discussion of how new plasma etch chemistries may be able to further enhance resist etch selectivity.