R. R. Kola

Stress relaxation in Mo/Si multilayer structures

The as‐deposited stress in sputtered, 75‐A‐period Mo/Si multilayers was measured to be approximately −350 MPa (compressive), and relaxed to approximately −150 MPa after thermal cycling to 200 °C. The multilayer period was found to decrease by 0.25 A as a result of thermal cycling, with only a slight decrease in peak soft‐x‐ray reflectance. The stress‐temperature behavior of individual Mo and Si films was also measured, and correlated with the multilayer behavior: stress relaxation in the multilayer is attributed to viscous flow associated with defect annihilation, occurring predominantly in the amorphous Si layers.

Stable low‐stress tungsten absorber technology for sub‐half‐micron x‐ray lithography

Tungsten is attractive for very large scale integrated device metallization and as absorber for x‐ray lithographic masks. To minimize distortions in an x‐ray mask, intrinsic stresses in the absorber films have to be low and reproducible. We present the results of a systematic study of the microstructure and stress of rf sputter‐deposited W films as a function of deposition parameters, using optical interferometry, scanning electron microscopy, transmission electron microscopy, and x‐ray diffraction. Rutherford backscattering spectrometry and Auger electron spectroscopy were used for the chemical analysis of the films. By controlling the nucleating phase and mobility of the adatoms, we have produced W films with low stresses (<±50 MPa). The low‐stress films have a 〈110〉 preferred orientation and a bimodal grain size distribution with large, elongated grains surrounded by small equiaxed grains. The lattice parameter and the argon content in W films increased with decreasing argon deposition pressure. It was...

Tungsten patterning for 1:1 x‐ray masks

A subtractive process to form subhalf micron, vertical‐walled patterns in half‐micron thick tungsten on x‐ray masks has been developed. Electron‐beam lithography was used to form resist patterns on a structure consisting of 300 A Cr on 5000 A W on 200 A Cr on an approximately 1 μm thick poly‐silicon or silicon nitride membrane. The Cr masking and etch‐stop layers above and below the W layer are required because the resist and membrane materials etch rapidly in fluorine based W etching plasmas. Chromium was chosen for these layers because it has a high selectivity in the W etch (≊40:1), is compatible with the W deposition process, and can be patterned in an O2–Cl2 plasma which does not etch W or the membrane materials. Helium backside cooling at a pressure from 1 to 5 Torr controls membrane temperature during all plasma processing steps. Pure CBrF3 or CHF3 etch W slowly while simultaneously depositing polymer which produces sloping profiles where the base of the feature is wider than the initial mask width...

Patterning of x‐ray masks using the negative‐acting resist P(SI‐CMS)

The copolymer of trimethylsilylmethyl methacrylate with chloromethylstyrene [P(SI‐CMS)] is a negative electron‐beam and deep‐UV resist which can withstand erosion in O2‐containing plasma environments [J. R. Maldonado, J. Electron. Mater. 19, 6699 (1990)]. Manipulation of its composition and molecular weight allows control of the etch resistance and radiation sensitivity properties. Methods have been developed to provide reproducible synthesis of P(SI‐CMS) with molecular weight and composition tailored to specific lithographic demands. For x‐ray mask patterning, the copolymer having a 90:10 mole ratio of SI:CMS [P(SI90‐CMS10)] and Mw between 30 000 and 41 000 g/mol has been found to provide an optimal combination of resist sensitivity, dry‐etching resistance, and pattern resolution. P(SI90‐CMS10) is used to image the Cr–W–Cr metallization layer of a monolithic x‐ray mask structure, by subtractive etching techniques [G. K. Celler et al., Appl. Phys. Lett. 59, 3105 (1991); G. K. Cellar et al., J. Vac. Sci. ...

Masks for x‐ray lithography with a point source stepper

We describe some key aspects of proximity x‐ray technology currently being developed at AT&T, from mask fabrication to wafer patterning. The masks are primarily based on polycrystalline Si membranes, 1 μm thick, which are formed directly on optically flat glass disks. A tungsten absorber layer is deposited on the membranes by radio‐frequency diode sputtering, with in situ stress control in the deposition chamber so that stresses ≤10 MPa are routinely achieved. Patterns are defined in an organosilicon negative resist, P(SI‐CMS), using an electron beam writing tool and a neural network based proximity correction algorithm. The patterns are transferred into metallic absorber layers by reactive ion etching in a parallel plate plasma system. Using the above procedure, we have fabricated masks with 0.25 μm features and also some test patterns with lines and spaces as small as 0.1 μm. X‐ray exposures were done with a Hampshire 5000P point source stepper, using AZ PF‐114 resist from Hoechst–Celanese.

Preliminary evaluation of a laser‐based proximity x‐ray stepper

This paper reports on the initial lithographic evaluation of a commercial 1:1 proximity stepper that uses a laser‐based plasma x‐ray source. Our preliminary tests have shown that 0.4‐ and 0.5‐μm lines and spaces can be printed consistently on Si wafers using a positive resist, which has a sensitivity of 6–10 mJ/cm2. Features smaller than 0.4 μm can be obtained, but the overlay accuracy of this system is targeted at 0.5‐μm design rules. The 3σ spread in linewidth is greater than 0.05 μm, however, the stepper contribution must be separated from the resist processing issues, reticle critical dimension variations, and scanning electron micrograph measurement precision.