Charles W. Jurgensen

Simulation of reactive ion etching pattern transfer

This paper describes a model that simulates etching profiles and process latitudes in glow‐discharge bombardment‐induced reactive‐etching processes. Numerical results are presented for the pattern‐transfer step in trilayer lithography, but this analysis is applicable to many other pattern‐transfer processes. The inputs to the interface‐evolution model described here are a kinetic model for the yield per incident energetic particle and a statistical mechanical model that relates the incident‐yield‐weighted angular distribution to the pressure, sheath thickness, and sheath voltage drop. The kinetic model is based on experimental evidence and assumes that the yield per bombarding particle is proportional to its energy. The resulting interface‐evolution equation is mathematically analogous to a free‐surface evolution equation in hydrodynamics. This convective partial differential equation is reduced to a coupled set of ordinary differential equations via the method of characteristics and solved numerically. M...

Sheath collision processes controlling the energy and directionality of surface bombardment in O2 reactive ion etching

The effect of sheath collision processes on the energy and directionality of surface bombardment in reactive ion etching is modeled. Although the methods used are generally applicable, all the numerical examples are for a low‐pressure high‐frequency oxygen plasma. Charge transfer is shown to be the dominant process controlling bombardment energies. The effect of momentum‐transfer collisions on ion bombardment energies is shown to be negligible. Equations are derived for the average energy of ions and neutrals, the average ion energy, the average neutral energy, and the ion energy distribution function. The ion drift velocity at a point in the sheath is related to the voltage distribution by an equation that provides a rigorous basis for a self‐consistent theory of the sheath voltage distribution. These equations are generally applicable to high‐frequency, low‐pressure plasmas where charge transfer is the dominant collision process. The angular distribution of energetic species is modeled using elastic sca...

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...

Nonlocal transport models of the self-consistent potential distribution in a plasma sheath with charge transfer collisions

Plasma sheaths are often assumed to be collision free; however, high‐voltage cathode sheaths are typically thicker than the mean free path for charge transfer collisions at pressures encountered in glow discharge processing equipment (greater than 10 mTorr). In this paper, the potential distribution in a plasma sheath is determined by solving Poisson’s equation self‐consistently using a kinetic theory nonlocal ion transport model for charge transfer collisions. The relationship between the potential distribution, ion flux, and thickness of a plasma sheath is presented for arbitrary values of the sheath thickness relative to the mean free path for charge transfer. The results may be used to estimate the ion flux from measurements of the sheath thickness and potential drop across the sheath. Ion energy distribution functions and a one‐parameter approximation to the numerically determined potential distribution are also presented. These results apply to rf discharges in a time‐averaged sense when the ion she...

Experimental tests of the steady‐state model for oxygen reactive ion etching of silicon‐containing polymers

A steady‐state model based on a silicon material balance has been proposed to predict the oxygen reactive‐ion‐etching resistance of organosilicon polymers. This model assumes that the rate determining step is sputtering of SiO2 film that forms on the surface of the organosilicon polymer. It predicts the etching rate of organosilicon polymers relative to the sputtering rate of SiO2 , based on the mass density of silicon in the polymer. The steady‐state etching rate of a silyl novolac polymer is accurately predicted by the model over a wide range of etching conditions. Silyl methacrylates etch at the predicted rate under high‐bombardment‐energy conditions typical of trilevel processing, but exceed the predicted rate under low‐bombardment‐energy conditions. Surface analysis shows that the SiO2 film thickness continues to increase with time under these conditions, invalidating the steady‐state approximation. Low silicon content (4.1 wt. %) polymers do not etch according to the model but form highly porous oxi...

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...

Kinetic theory of bombardment induced interface evolution

An interface evolution equation has been formulated to describe bombardment‐induced etching by an axisymmetric angular distribution of energetic particles where the yield per incident particle is assumed to be a function of its energy and its angle relative to the surface normal. These assumptions result in a nonlinear integro differential equation, but this equation reduces to a partial differential equation in several important special cases. At points that are not shadowed by a remote part of the surface, the interface evolution equation reduces to a nonlinear hyperbolic conservation law. Such equations have been applied to bombardment‐induced etching by a monodirectional beam with angle‐dependent yields; however, this form of equation applies more generally to raised isolated convex regions (e.g., etching masks) regardless of the angular distribution of the incident particles or the angle dependence of the yield. The essential qualitative feature of the solution in these cases is the spontaneous evolu...

Experimental Tests Of The Steady-State Model For Oxygen Reactive Ion Etching Of Silicon-Containing Polymers

A steady-state model has been proposed to predict the oxygen reactive-ion-etching resistance of organosilicon polymers. This model is based on a Silicon material balance and the assumption that a protective Si02 film forms and reaches a steady-state thickness on the surface of the polymer. At steady-state, the rate determining step is sputtering of the SiO2 film. This model predicts that the steady-state etching rate is proportional to the sputtering rate of Si02 and inversely proportional to the mass density of silicon in the polymer. It also predicts that the etching rate is independent of other chemical or physical properties of the material. This model accurately predicts the etching behavior of a silyl novolac polymer over a wide range of etching conditions. Two silyl methacrylates etch at the predicted rate under conditions typical of trilevel processing, but exceed the predicted rate under conditions where the average bombardment energy is lower. Surface analysis shows that the steady-state approximation is not valid for the methacrylates under these etching conditions; the oxide thickness continues to increase with time, even though a constant etching rate is achieved. Polymers with very low silicon content do not etch according to the model but form highly porous oxides that continuously accumulate on the surface of the polymer.

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.