A. E. Wendt

Control of ion energy distribution at substrates during plasma processing

Control of ion energy distribution functions (IEDF) at the substrate during plasma processing is achieved using a specially tailored voltage waveform for substrate bias, consisting of a short voltage spike in combination with a slow ramp. A much narrower IEDF is possible compared to the conventional approach of applying a sinusoidal waveform to the substrate electrode. Measurements in a helicon plasma combined with a time-dependent spherical-shell plasma fluid model demonstrate the benefits of this method in producing a narrow IEDF of precisely controllable energy, independent of ion mass.

High-density plasma sources

Publisher Summary The chapter discusses the challenges for ionized physical vapor deposition plasma source design. Ionized physical vapor deposition (I-PVD), in comparison with other process steps for ULSI (ultra large-scale integration) fabrication, presents some interesting and unique challenges in the selection of a plasma source. One general approach to enhancing the ionization of the sputtered material is to increase the distance between the sputter target and substrate in order to generate a “high-density plasma” in the space created. Several types of high-density plasma sources are employed for I-PVD processes and are described in this chapter. Most approaches to I-PVD use some form of DC magnetron discharge as a source of sputtered metal. Inductively coupled plasmas (ICP) and electron cyclotron resonance (ECR) plasmas are used as secondary plasma sources to enhance the ionization of metal atoms sputtered from DC magnetron sources. Finally, the hollow cathode magnetron source, developed especially for I-PVD, combines sputtering and ionization in a single source.

Plasma etch removal of poly(methyl methacrylate) in block copolymer lithography

Polystyrene-block-poly(methyl methacrylate), (PS-b-PMMA) diblock copolymer is a promising lithography alternative for nanometer scale features. The two components segregate into nanoscale domains when the polymer solution is spun on to form a thin film and annealed above the glass transition temperatures of both components. Preferential removal of PMMA domains through plasma etching to leave behind a PS mask for subsequent etching of underlying layers is the focus of this work. The quality of the PS mask is characterized by the thickness and lateral dimension of the PS structures after removal of the PMMA, as well as the smoothness of their surfaces. The effects of different plasma chemistries including O2, Ar/O2, Ar, CF4, and CHF3/O2 on etch selectivity and surface/sidewall roughness for PS and PMMA have been characterized. Ar/O2 produced the overall best results for the range of conditions studied.

Pattern transfer using poly(styrene-block-methyl methacrylate) copolymer films and reactive ion etching

Self-assembly block copolymers have drawn a lot of attention for its great potential on critical dimension (CD) control and line-edge roughness (LER) reduction, which become more and more crucial as the CD of transistors is only tens of nanometers nowadays. In this study, lamellar-forming poly(styrene-b-methyl methacrylate) copolymer which fabricates line patterns was chosen for its ability to provide higher aspect ratio and vertical sidewall profile in template stage, thus more suitable for the following etching process to substrate. A dry plasma etching process using pure oxygen and pure argon plasma as example chemical etching gas and physical etching gas, respectively, was studied. Etching selectivity and lateral etch rate, which are responsible for the final template height and CD loss, had been characterized on a capacitive reactive ion etching tool. The templates formed by the proposed process had high aspect ratios, excellent pattern fidelity, and low LER values. The PS lateral etch rate was small...

Electron‐density and energy distributions in a planar inductively coupled discharge

Electron‐density and electron energy distribution functions (EEDFs) are measured in a 20‐cm‐diam by 14‐cm‐long cylindrical, inductively coupled plasma source driven by fields from a planar, spiral coil at 13.6 MHz. Radio‐frequency (rf) ‐filtered Langmuir probes are used to obtain spatial profiles of electron population characteristics in argon at powers and pressures of interest for etching and plasma‐assisted deposition (1–100 mT). Electron densities range from 1010 to 1012 cm3 with 100–500 W of rf power and peak on axis in the center of the cylindrical volume. The EEDFs show that the observed average electron energy varies by 1–2 eV spatially, with the highest values of average energy occurring at those regions of strongest rf electric field. The EEDF measurements also reveal a significant population of cold electrons trapped in a potential well at the location of peak electron density. From these spatial measurements, spatial estimates of conductivity and ionization rate are deduced.

Measurement of metastable and resonance level densities in rare-gas plasmas by optical emission spectroscopy

Excitation and ionization of atoms out of the 4 energy levels of the excited np5(n + 1)s configuration of rare gases play an important role in many low temperature rare-gas plasmas. We compare two optical methods for measuring the number densities of atoms in these excited levels in an inductively coupled plasma under a variety of operating conditions (600 W, 1–25 mTorr). The first method is a standard white light absorption technique, whereas the second method exploits changes in the effective branching fractions of np5(n + 1)p → np5(n + 1)s emissions brought about by radiation trapping of atoms in np5(n + 1)s levels. The branching fraction method was found to produce results that agree well with the direct white light absorption method for both argon and neon plasmas using little more than a low-resolution spectrum of the plasma glow.

Optical emission measurements of electron energy distributions in low-pressure argon inductively coupled plasmas

Optical modeling of emissions from low-temperature plasmas provides a non-invasive technique to measure the electron energy distribution function (EEDF) of the plasma. While many models assume the EEDF has a Maxwell?Boltzmann distribution, the EEDFs of numerous plasma systems deviate significantly from the Maxwellian form. In this paper, we present an optical emission model for the Ar(3p54p ? 3p54s) emission array which is capable of capturing details of non-Maxwellian distributions. Our model combines previously measured electron-impact excitation cross sections with Ar(3p54s) number density measurements and emission spectra. The model also includes corrections for radiation trapping of the Ar(3p54p ? 3p54s) emission lines. Results obtained with this optical technique are compared with corresponding Langmuir probe measurements of the EEDF for Ar and Ar/N2 inductively coupled plasma systems operating under a wide variety of source conditions (1?25?mTorr, 20?1000?W, %N2 admixture). Both the optical emission method and probe measurements indicate the EEDF shapes are Maxwellian for low electron energies, but with depleted high energy tails.

Profiling and modeling of dc nitrogen microplasmas

This article explores electric current and field distributions in dc microplasmas, which have distinctive characteristics that are not evident at larger dimensions. These microplasmas, which are powered by coplanar thin-film metal electrodes with 400-μm minimum separations on a glass substrate, are potentially useful for microsystems in both sensing and microfabrication contexts. Experiments in N2 ambient show that electron current favors electrode separations of 4 mm at 1.2 Torr, reducing to 0.4 mm at 10 Torr. The glow region is confined directly above the cathode, and within 200–500 μm of its lateral edge. Voltage gradients of 100 kV/m exist in this glow region at 1.2 Torr, increasing to 500 kV/m at 6 Torr, far in excess of those observed in larger plasmas. Numerical simulations indicate that the microplasmas are highly nonquasineutral, with a large ion density proximate to the cathode, responsible for a dense space-charge region, and the strong electric fields in the glow region. It is responsible for ...

Nonlocal electron kinetics in a low‐pressure inductively coupled radio‐frequency discharge

The Boltzmann equation for electrons is analyzed for a low‐pressure inductively coupled rf discharge in argon driven by a planar coil. Spatially resolved probe measurements of the electron distribution function (EDF) indicate that the total energy of electrons is an argument of the EDF. Pressure dependence of the light emission distribution is explained on the basis of nonlocal electron kinetics.

Arbitrary substrate voltage wave forms for manipulating energy distribution of bombarding ions during plasma processing

The energy distribution of ions (IED) bombarding a substrate during plasma etching has demonstrated effects on etch selectivity for integrated circuit fabrication. Accurate control of the IED is desired to better understand the nature of plasma–surface interaction and to control process outcomes. IED control can be achieved by tailoring the wave form shape of an rf bias applied to the substrate, using a programmable wave form generator in combination with a power amplifier. Due to the frequency dependence of the amplifier gain and the impedance of the plasma in contact with the substrate, it is not practical to predict the shape of the input wave form needed to produce a desired result at the substrate. Introduced here is an iterative approach using feedback control in the frequency domain to produce arbitrary wave form shapes at the substrate. A fast Fourier transform (FFT) of the substrate wave form is compared, one frequency at a time, with the FFT of a desired target wave form, to determine adjustments needed at the generator. This iterative procedure, which is fully automated and tested for several target wave form shapes, is repeated until the substrate wave form converges to the targeted shape.